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Originally appearing in Volume V11, Page 513 of the 1911 Encyclopedia Britannica.
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GASTROPODA, the second of the five classes of animals constituting the phylum Mollusca. For a discussion of the relationship of the Gastropoda to the remaining classes of the phylum, see MOLLUSCA. The Gastropoda are mainly characterized by a loss of symmetry, produced by torsion of the visceral sac. This torsion may be re-solved into two successive movements. The first is a ventral flexure in the antero-posterior or sagittal plane; the result of this is to approximate the two ends of the alimentary canal. In development, the openings of the mantle-cavity and the anus are always originally posterior; later they are brought forward ventrally. During this first movement flexure is also produced by the coiling of the visceral sac and shell; primitively the latter was bowl-shaped; but the ventral flexure, which brings together the two extremities of the digestive tube, gives the visceral sac the outline of a more or less acute cone. The shell necessarily takes this form also, and then becomes coiled in a dorsal or anterior plane—that is to say, it becomes exogastric. This condition may be seen in embryonic Patellidae, Fissurellidae and Trochidae (fig. 1, A), and agrees with the method of coiling of a mollusc without lateral torsion, such as Nautilus. But ultimately the coil becomes ventral or endogastric, in consequence of the second torsion movement then apparent. A. B. From Lankester's Treatise on Zoology. A, Nearly symmetrical larva f, Foot. (veliger). op, Operculum. B, A stage 11 hours later than A. pac, Pallial cavity. C, A stage 31 hours later than B. ve, Velum. The shell is represented as fixed, while the head and foot rotate from left to right. In reality the head and foot are fixed and the shell rotates from right to left. The second movement is a lateral torsion of the visceral mass, the foot remaining a fixed point; this torsion occurs in a plane approximately at right angles to that of the first movement, and carries the pallial aperture and the anus from behind forwards. If, at this moment, the animal were placed with mouth and ventral surface turned towards the observer, this torsion carries the circumanal complex in a clockwise direction (along the right side in dextral forms) through 18o° as compared with its primitive condition. The (primitively) right-hand organs of the complex thus become left-hand, and vice versa. The visceral commissure, while still surrounding the digestive tract, becomes looped; its right half, with its proper ganglion, passes to the left side over the dorsal face of the alimentary canal (whence the name supra-intestinal), while the left half passes below towards the right side, thus originating the name infra-intestinal given to this half and to its ganglion. Next, the shell, the coil of which was at first exogastric, being also included in this rotation through 18o°, exhibits an endogastric coiling (fig. 1, B, C). This, however, is not generally retained in one plane, and the spire projects, little by little, on the side which was originally left, but finally becomes right (in dextral forms, with a clockwise direction, if viewed from the side of the spire; but counter-clockwise in sinistral forms). Finally, the original symmetry of the circumanal complex vanishes; the anus leaves the centre of the pallial cavity and passes towards the right side (left side in sinistral forms) ; the organs of this side become atrophied and disappear. The essential feature of the asymmetry of Gastropoda is the atrophy or disappearance of the primitively left half of the circumanal complex (the right half in sinistral forms), including the gill, the auricle, the osphradium, the hypobranchial gland and the kidney. In dextral Gastropods the only structure found on the topographically right side of the rectum is the genital duct. But this is not part of the primitive complex. It is absent in the most primitive and symmetrical forms, such as Haliotis and Pleurotomaria. Origin-ally the gonads opened into the kidneys. In the most primitive existing Gastropods the gonad opens into the right kidney (Patellidae, Trochidae, Fissurellidae). The gonaduct, therefore, is derived fromthe topographically right kidney. The transformation has been actually shown to take place in the development of Paludina. In a dextral Gastropod the shell is coiled in a right-handed spiral from apex to mouth, and the spiral also A B projects to the right of the median plane of the animal. When the shell is sinistral the asymmetry of the organs is usually reversed, and there is a complete situs inversus viscerum, the direction of the spiral of the shell corresponding to the position of the organs of the body. Triforis, Physa, Clausilia are examples of sinistral Gastropods, but reversal also occurs as an individual variation among forms normally dextral. But there are forms in which the involution is " hyperstrophic," that is to say, the turns of the spire projecting but slightly, the spire, after flattening out gradually, finally becomes re-entrant and transformed into a false umbilicus; at the same time that part which corresponds to the umbilicus of forms with a normal coil projects and constitutes a false spire; the coil thus appears to be sinistral, although the asymmetry remains dextral, and the coil of the operculum (always the opposite to that of the shell) sinistral (e.g. Lanistes among Streptoneura, Limacinidae among Opisthobranchia). The same, mulatis mutandis, may occur .in sinistral shells. The problem of the causes of the torsion of the Gastropod body has been much discussed. E. R. Lankester in the ninth edition of this work attributed it to the pressure of the shell and visceral hump towards the right side. He referred also to the nautiloid shell of the larva falling to one side. But these are two distinct processes. In the larva a nautiloid shell is developed which is coiled exogastrically, that is, dorsally, and the pallial cavity is posterior or ventral (fig. 2, C) : the larva therefore resembles Nautilus in the relations of body and shell. The shell then rotates towards the left side through 18o°, so that it becomes ventral or endogastric (fig. 2, D). The pallial cavity, with its organs, is by this torsion moved up the right side of the larva to the dorsal surface, and thus the left organs become right and vice versa. In the subsequent growth of the sub-intestinal) visceral ganglion. cerg, Cerebral ganglion. pig, Pleural ganglion. pedg, Pedal ganglion. abg, Abdominal ganglion. butt, Buccal mass. W, Wooden arc representing the base-line of the wall of the visceral hump. x, x', Pins fastening the elastic cord (representing the vis- ceral nerve loop) to W_ the shell the spire comes to project on the right side, which was originally the left. Neither the rotation of the shell as a whole nor its helicoid spiral coiling is the immediate cause of the torsion of the body in the individual, for the direction of the torsion is indicated in the segmentation of the ovum, in which there is a complete many cases be useful. Lavage or washing out of the stomach with weak alkaline solutions has been used with marked success in the treatment of chronic gastritis. Of medicinal agents, bismuth, arsenic; nux vomica, and the mineral acids are all of acknow- P C. From Lankester's Treatise on Zoology. Embryo without flexure. Embryo with ventral flexure of the intestine. C, Embryo with ventral flex- ure and exogastric shell. D, Embryo with lateral tor- sion and an endogastric shell. a, Anus. f, Foot. m, Mouth. pa, Mantle. pac, Pallial cavity. ve, Velum. A, B, a A, Unrotated ancestral condition. B, Quarter-rotation. C, Complete semi-rotation(the limit). an, Anus. in, rn, Primarily left nephridium and primarily right nephridium. leg, Primarily left (subsequently the sub-intestinal) visceral ganglion. rvg, Primarily right(subsequently reversal of the cleavage planes in sinistral as compared with dextral forms. The facts, however, strongly suggest that the original cause of the torsion was the weight of the exogastric shell and visceral hump, which in an animal creeping on its ventral surface necessarily fell over to one side. It is not certain that the projection of the spire to the originally left side of the shell has anything to do with the falling over of the shell to that side. The facts do not support such a suggestion. In the larva there is no projection at the time the torsion takes place. In some forms the coiling disappears in the adult, leaving the shell simply conical as in Patellidae, Fissurellidae, &c., and in some cases the shell is coiled in one plane, e.g. Planorbis. In all these cases the torsion and asymmetry of the body are unaffected. The characteristic torsion attains its maximum effect among the majority of the Streptoneura. It is followed in some specialized Heteropoda and in the Euthyneura by a torsion in the opposite direction, or detorsion, which brings the anus farther back and untwists the visceral commissure (see Euthyneura, below). This conclusion has shown that the Euthyneura do not represent an archaic form of Gastropoda, but are themselves derived from streptoneurous forms. The difference between the two sub-classes has been shown to be slight; certain of the more archaic Tectibranchia (Actaeon) and Pulmonata (Chilina) still have the visceral commissure long and not untwisted. The fact that all the Euthyneura are hermaphrodite is not a fundamental difference; several Streptoneura are so, likewise Valvata, Oncidiopsis, Marsenina, Odostomia, Bathysciadium, Entoconcha: Classification.—The class Gastropoda is subdivided as follows: Sub-class I. Streptoneura. Order 1. Aspidobranchia. Sub-order 1. Docoglossa. 2. Rhipidoglossa. Order 2. Pectinibranchia. Sub-order 1. Taenioglossa. Tribe 1. Platypoda. 2. Heteropoda. Sub-order 2. Stenoglossa. Tribe 1. Rachiglossa. 2. Toxiglossa. Sub-class II. Euthyneura. Order I. Opisthobranchia. Sub-order 1. Tectibranchia. Tribe 1. Bullomorpha. 2. Aplysiomorpha. 3. Pleurobranchomorpha. Sub-order 2. Nudibranchia. Tribe 1. Tritoniomorpha. 2. Doridomorpha. 3. Eolidomorpha. 4. Elysiomorpha. Order 2. Pulmonata. Sub-order 1. Basommatophora. 2. Stylommatophora. Tribe i. Holognatha. 2. Agnatha. 3. Elasmognatha. 4. Ditremata. Sub-Class I.—STREFTONEURA In this division the torsion of the visceral mass and visceral commissure is at its maximum, the latter being twisted into a figure of eight. The right half of the commissure with its ganglion is supra-intestinal, the left half with its ganglion infra-intestinal. In some cases each pleural ganglion is connected with the opposite branch of the visceral commissure by anastomosis with the pallial nerve, a condition which is called dialyneury; or there may be a direct connective from the pleural ganglion to the visceral ganglion of the opposite side, which is called zygoneury. The head bears only one pair of tentacles. The radular teeth are of several different kinds in each transverse row. The heart is usually posterior to the branchia (proso-branchiate). The sexes are usually separate. The old division into Zygobranchia and Azygobranchia must be abandoned, for the Azygobranchiate Rhipidoglossa have much greater affinity to the Zygobranchiate. Haliotidae and Fissurellidae than to the Azygobranchia in general. This is shown by the labial commissure and pedal cords of the nervous system, by the opening of the gonad into the right kidney, and by other points. Further, the Fleur atom ariidae have been discovered to possess two branchiae. The sub-class is now divided into two orders: the Aspidobranchia in which the branchia or ctenidium is bipectinate and attached only at its base, and the Pectinibranchia in which the ctenidium is monopectinate and attached to the mantle throughout its length. Order 1. RANG ma.—These are the most primitive Gastropods, retaining to a great degree the original symmetry of the z x, y, The median antero-posterior special pallial growths, com- axis. parable with those of Pleuro- a, Cephalic tentacle. phyllidia). b, Plantar surface of the foot. g, The branchial efferent vessel. c, Free edge of the shell. h, Factor of the branchial ad- d, The branchia) efferent vessel vehent vessel. carrying aerated blood to the i, Interspaces between the muss auricle, and here interrupting cular bundles of the root of the circlet of gill lamellae. the foot, causing the separate e, Margin of the mantle-skirt. areae seen in fig. g, c. f, Gill lamellae (not ctenidia, but organs of the pallial complex, having two kidneys, in some cases two branchiae, and two auricles. The gonad has no accessory organs and except in Neri- tidae no duct, but discharges into the right kidney. Forms adapted to terrestrial life and to aerial respiration occur in various divisions of Gastropods, and do not constitute a single homogeneous group. Thus the Helicinidae, which are terrestrial, are now placed among the Aspidobranchia. In these there are neither branchia nor osphradium, and the pallial chamber which retains its large opening serves as a lung. De-generation of the shell occurs in some members of the order. It is largely covered by the mantle in some Fissurellidae, is en- FIG. 5.—Dorsal surface of the tirely internal in Pupili¢ Limpet removed from its shell and de-and absent in Titiscaniidae. prived of its black pigmented epithe- The common limpet is a hum; the internal organs are seen specially . interesting and through the transparent body-wall. abundant example of the (Lankester.) more primitive Aspido- branchia. The foot of the e Muscular bundles forming the root limpet is a nearly circular of the foot, and adherent to the disk of muscular tissue; in shell. front, projecting from and e, Free mantle-skirt. [same. raised above it, are the head em, Tentaculiferous margin of the and neck (figs. 4, 13). The i, Smaller (left) nephridium. visceral hump forms a low k, Larger (right) nephridium. conical dome above the sub- 1, Pericardium. [cardium. circular foot, and standing Is, Fibrous septum, behind the pen-out all round the base of this n, Liver. dome so as completely to ant, Intestine. overlap the head and foot, ecr, Anterior area of the mantle-skirt is the circular mantle-skirt. over-hanging the head (cephalic The depth of free mantle- hood). skirt is greatest in front, where the head and neck are covered in by it. Upon the surface of the Visceral dome, and extending k to the edge of the free mantle-skirt, is the conical shell. When the shell is taken away (best effected by immersion in hot water) the surface of the visceral dome is found to be covered by a black-coloured epithelium, which may be removed, enabling the observer to note the position of some organs lying below the transparent in-tegument (fig. 5). The muscular columns (c) attaching the foot to the shell form a ring incomplete in front, external to which is the free mantle-skirt. The limits of the large area formed by the flap over the head and neck (ecr) can be traced, k and we note the anal papilla showing through sub-pallial space. Close to this the small renal organ (i, mediad) and the larger renal organ (k, to the right and posteriorly) are seen, also the pericardium (l) and a coil of the intestine (Mt) em-bedded in the compact liver. On cutting away the anterior part of the mantle-skirt so as to expose the sub-pallial chamber in the region of the neck, we find the right and left renal papillae (discovered by Lankester in 1867) on either side of the anal papilla (fig. 6), but no gills. If a similar examination be made of the allied genus Fissurella (fig. 17, d), we find right and left of the two renal apertures a right and left gill-plume or ctenidium, which here as in Haliotis and Pleurotomarsa retain their original paired condition. In Patella no such plumes exist, but right and left of the neck are seen a pair of minute oblong yellow bodies (fig. 6, d), which were originally described by Lankester as orifices possibly connected with the evacuation of the generative products. On account of their position they were termed by him the " capito-pedal orifices," being placed near the junction of head and foot. J. W. Spengel has, however, in a most ingenious way shown that these bodies are the representatives of the typical pair of ctenidia, here reduced to a mere rudiment. Near to each rudimentary ctenidium Spengel has discovered an olfactory patch or osphradium (consisting of modified epithelium) and an olfactory nerve-ganglion (fig. 8). It will be remembered that, according to Spengel, the osphradium of mollusca is definitely and intimately related to the gill-plume or ctenidium, being always placed near the base of that organ; fur- ther, Spengel has shown that the nerve-supply of k this olfactory organ is always derived from the visceral loop. Accordingly, the nerve-supply affords a means of testing the conclusion that we have in Lankester's capito-pedal bodies the rudimentary ctenidia. The accompanying dia- grams (figs. 9, 10) of the nervous systems of Patella and of Haliotis, as determined by Spengel, show the identity in the origin of the nerves passing from the visceral loop to Spengel's olfactory ganglion of the Limpet, and that of the nerves which pass from the visceral loop of Haliotis to the olfactory patch or osphradium, which lies in immediate relation on the right and on the left side to the right and left gill-plumes (ctenidia) respectively. The same diagrams serve to demonstrate the streptoneurous condition of the visceral loop in Aspidobranchia. Thus, then, we find that the limpet possesses a symmetrically disposed pair of ctenidia in a rudimentary condition, and justifies its position among Aspidobranchia. At the same time it possessesa totally distinct series of functional gills, which are not derived from the modification of the typical molluscan ctenidium. These gills are in the form of delicate lamellae (fig. 4, f), which form a series extending completely round the inner face of the depending mantle- B, Surface view of a rudimentary ctenidium of Patella excised and viewed as a transparent object. (Lankester.) skirt. This circlet of gill-lamellae led Cuvier to class the limpets as Cyclobranchiata, and, by erroneous identification of them with the series of metamerically repeated ctenidia of Chiton, to associate the latter mollusc with the former. The gill-lamellae of Patella are processes of the mantle comparable with the plait-like folds often observed on the roof of the branchial chamber in other Gastropoda (e.g. Buccinum and Haliotis). They are termed pallial gills. The only other molluscs in which they are exactly represented are the curious Opisthobranchs Phyllidia and Pleurophyllidia (fig. 55). In these, as in Patella, the typical ctenidia are aborted, and the branchial function is assumed by close-set lamelliform processes arranged in a series beneath the mantle-skirt on either side of the foot. In fig. 4, d, the large branchial vein of Patella bringing blood from the gill-series to the heart is seen; where it crosses the series of lamellae there is a short interval devoid of lamellae. The heart in Patella consists of a single auricle (not two as iii Haliotis and Fissurella) and a ventricle; the former receives the blood from the branchial vein, the latter distributes it through a large aorta which soon leads into irregular blood-lacunae. The existence of two renal organs in FIG. 9.-Nervous sys-Patella, and their relation to the pen- tern of Patella; the viscardium (a portion of the coelom) is coral loop is lightly important. Each renal organ is a sac shaded; the buccal lined with glandular epithelium (ciliated ganglia are omitted. cell, with concretions) communicating (After Spengel.) with the exterior by its papilla, and by ce, Cerebral ganglia. a narrow passage with the pericardium. c'e, Cerebral commissure. The connexion with the pericardium of pl, Pleural ganglion. the smaller of the two renal organs was pe, Pedal ganglion. demonstrated by Lankester in 1867, at a p'e, Pedal nerve. time when the fact that the renal organ s,s', Nerves (right and of the Mollusca, as a rule, opens into the left) to the mantle. pericardium, and is therefore a typical o, Olfactory ganglion, nephridium, was not known. Subsequent connected by nerve investigations carried on under the direc- to the streptoneur- tion of the same naturalist have shown ous visceral loop. that the larger as well as the smaller renal sac is in communication with the pericardium. The walls of the renal sacs are deeply plaited and thrown into ridges. Below the surface these walls are excavated with blood-vessels, so that the sac is practically a series-of blood-vessels covered with renal epithelium, and forming n • Cephalic tentacle. Foot. • Muscular substance forming the root of the foot. The capito-pedal organs of Lankester (=rudimentary ctenidia). • Mantle-skirt. Papilla of the larger nephridium. • Anus. Papilla of the smaller nephridium. Smaller nephridium. Larger nephridium. Pericardium. Cut edge of the mantle-skirt. • Liver. Snout. c ff —t a meshwork within a space communicating with the exterior. The larger renal sac (remarkably enough, that which is aborted in other shaded; the buccal ganglia are omitted. (After Spengel.) ce, Cerebral ganglion. s, s', Right and left mantle, The f used pleural and pedal nerves. [of same. ganglia. ab, Abdominal ganglion or site pe, The right pedal nerve. o, o, Right and left olfactory, The cerebro-pleural con- ganglia and osphradia re- nective. [tive. ceiving nerve from vis-, The cerebra-pedal connec- ceral loop. Anisopleura) extends between the liver and the integument of the visceral dome very widely. It also bends round the liver as shown ksi in fig. 12, and forms a large sac on half of the upper surface of the muscular mass of the foot. Here it lies close upon the genital body (ovary or testis), and in such intimate relationship with it that,, when ripe, the gonad bursts into the renal sac, and its products are carried to the exterior by the papilla on the right side of the anus q, Intestine in transverse sec- br.a, Branchial advehent vessel tion. (artery). r, Lingual sac (radular sac). br.v, Branchial efferent vessel rd, Radula. (vein). s, Lamellated stomach. bv, Blood-vessel. 1, Salivary gland. odm, Muscles and cartilage of u, Duct of same. the odontophore. v, Buccal cavity cor, Heart within the pericar- w, Gonad. dium. (Robin, Dail). This fact led Cuvier erroneously to the belief that a duct existed leading from the gonad to this papilla. The position of the gonad, best seen in the diagrammatic section (fig. 13), is, as in other Aspidobranchia, devoid of a special duct communicating with the exterior. This condition, probably an archaic one, distinguishes the Aspidobranchia from other Gastropoda. The digestive tract of Patella offers some interesting features. The odontophore is powerfully developed; the radular sac is extra-ordinarily long, lying coiled in a space between the mass of the liver and the muscular foot. The radula has 16o rows of teeth with twelve teeth in each row. Two pairs of salivary ducts, each leading from a salivary gland, open into the buccal chamber. The oesophagus leads into a remarkable stomach, plaited like the manyplies of a sheep, and after this the intestine takes a very large number of turns em-bedded in the yellow liver, until at last it passes between the two renal sacs to the anal papilla. A curious ridge (spiral ? valve) kS.L f, Papilla of the larger neph- ridium. g, Anal papilla with rectum leading from it. h, Papilla of the smaller neph- ridium, which is only repre- sented by dotted outlines. 1, Pericardium indicated by a FIG. 11.-Nervous system of dotted outline—at its right Fissurella. (From Gegenbaur, side are seen the two reno- after Jhering.) pericardial pores. p1, Pallial nerve. ff, The sub-anal tract of the large p, Pedal nerve. nephridium given off near its A, Abdominal ganglia in the strep- papilla and seen through the toneurous visceral commissure, unshaded smaller nephri- with supra- and sub-intestine dium. ganglion on each side. ks.a, Anterior superior lobe of B, Buccal ganglia. the large nephridium. C, C, Cerebral ganglia. ks.1, Left lobe of same. es, Cerebral commissure. ks.p, Posterior lobe of same. o, Otocysts attached to the cere- ks.i, Inferior sub-visceral lobe bro-pedal connectives. of same. a3 / a, Large or external or right f, Manyplies. renal organ. g, Epithelium of the dorsal sur- ab, Narrow process of the same face. running below the intestine h, Renal epithelium lining the and leading by k into the renal sacs. pericardium. i, Aperture connecting the b, Small or median renal organ. small sac with the peri- c, Pericardium. cardium. d, Rectum. k, Aperture connecting the large e, Liver. sac with the pericardium. which secretes a slimy cord is found upon the inner wall of the intestine. The general structure of the Molluscan intestine has not been sufficiently investigated to render any comparison of this structure of Patella with that of other Mollusca possible. The eyes of the limpet deserve mention as examples of the most primitive kind of eye in the Molluscan series. They are found one on each. cephalic tentacle, and are simply minute open pits or depressions of the epidermis, the epidermic cells lining them being pigmented and connected with nerves (compare fig. 14, art. CEPHALOPODA). The limpet breeds upon the southern English coast in the early part of April, but its development has not been followed. It has simply been traced as far as the formation of a diblastula which acquires a ciliated band, and becomes a nearly spherical trochosphere. It is probable that the limpet takes several years to attain full growth, and during that period it frequents the same spot, which becomes gradually sunk below the surrounding surface, especially if the rock be carbonate of lime. At low tide the limpet (being a strictly intertidal organism) is exposed to the air, and (according to trustworthy observers) quits its attachment and walks away in search of food (minute encrusting algae), and then once more returns to the identical spot, not an inch in diameter, which belongs, as it were, to it. Several million limpets—twelve million in Berwickshire alone—are annually used on the east coast of Britain as bait. Sub-order i. Docoglossa.—Nervous system without dialyneury. Eyes are open invaginations without crystalline lens. Two osphradia present but no hypobranchial glands nor operculum. Teeth of radula beam-like, and at most three marginal teeth on each side. Heart has only a single auricle, neither heart nor pericardium traversed by rectum. Shell conical without spire. Fam. i.—Acmaeidae. A single bipectinate ctenidium on left side. Acmaea, without pallial branchiae, British. Scurria, with pallial branchiae in a circle beneath the mantle. Fam. z.—Tryblidiidae. Muscle scar divided into numerous impressions. Tryblidium, Silurian. Fam. 3.—Patellidae. No ctenidia but pallial branchiae in a circle between mantle and foot. Patella, pallial branchiae forming a complete circle, no epipodial tentacles, British. Ancistromesus, radula with median central tooth. Nacelle, epipodial tentacles present. Helcion, circlet of branchiae interrupted anteriorly, British. Fam. 4.—Lepetidae. Neither ctenidia nor pallial branchiae. Lepeta, without eyes. Pilidium. Propilidium. Fam. 5.—Bathysciadidae. Hermaphrodite; head with append-age on right side; radula without central tooth. Balhysciadium, abyssal. Sub-order 2. RHIPIDOGL0ssA.-Aspidobranchia with a palliovisceral anastomosis (dialyneurous); eye-vesicle closed, with crystalline lens; ctenidia, osphradia and hypobranchial glands paired or single. Radula with very numerous marginal teeth arranged like the rays of a fan. Heart with two auricles; ventricle traversed by the rectum, except in the Helicinidae. An epipodial ridge on each side of the foot and cephalic expansions between the tentacles often present. Fam. i.—Pleurotomariidae. Shell spiral; mantle and shell with an anterior fissure; two ctenidia; a horny operculum. Pleurotomaria, epipodium without tentacles. Genus includes several hundred extinct species ranging from the Silurian to the Tertiary. Five living species from the Antilles, Japan and the Moluccas. Moluccan species is 19 cm. in height. Fam. 2.—Bellerophontidae. 300 species, all fossil, from Cambrian to Trias. Fam. 3.—Euomphalidae. Also extinct, from Cambrian to Cretaceous. Fam. 4.—Haliotidae. Spire of shell much reduced; two bipectinate ctenidia, the right being the smaller; no operculum. Haliotis. Fam. 5.— Velainiellidae, an extinct family from the Eocene. Fam. 6.—Fissurellidae. Shell conical; slit or hole in anterior part of mantle; two symmetrical ctenidia; no operculum. Emarginula, mantle and shell with a slit, British. Scutum, mantle split anteriorly and reflected over shell, which has no slit. Puncturella, mantle and shell with a foramen in front of the apex, British. Fissurella, mantle and shell perforated at apex, British. Fam. 7.—Cocculinidae. Shell conical, symmetrical, without slit or perforation. Cocculina, abyssal. Fam. 8.—Trochidae. Shell spirally coiled; a single ctenidium; eyes perforated; a horny operculum; lobes between the tentacles. Trochus, shell umbilicated, spire pointed and prominent, British. Monodonta, no jaws, spire not prominent, no umbilicus, columella toothed. Gibbula, with jaws, three pairs of epipodial cirri without pigment spots at their bases, British. Margarita, five to seven pairs of epipodial cirri with a pigment spot at base of each. Fam. 9.—Stomatellidae. Spire of shell much reduced; a single ctenidium. Stomatella, foot truncated posteriorly, an oper- b a, Cephalic tentacle. b, Foot. [plume. d, Left (archaic right) gill- e, Reflected mantle-flap. fi, The fissure or hole in the mantle-flap traversed by the longitudinal incision. f, Right (archaic left) nephri- dium's aperture. g, Anus. h, Left (archaic right) aperture of nephridium. p, Snout. culum present, no epipodial tentacles. Gena, foot elongated posteriorly, no operculum. Fam. io.—Delphinulidae. Shell spirally coiled; operculum horny; intertentacular lobes absent. Delphinula. Fam. i i.—Liotiidae, shell globular, margin of aperture thickened. Liotia. Fam. 12.e--Cyclostrematidae. Shell flattened, umbilicated; foot anteriorly truncated with angles produced into lobes. Cyclostrema. Teinosloma. Fam. 13.—Trochonematidae. All extinct, Cambrian to Cretaceous. Fam. 14.—Turbinidae. Shell spirally coiled; epipodial tentacles present; operculum thick and calcareous. Turbo. Astralium. Molleria. Cyclonema. Fam. 15. Phasianellidae. Shell not nacreous, without umbilicus, with prominent spire and polished surface. Phasianella. Fam: 16.—Umboniidae. Shell flattened, not umbilicated, generally smooth; operculum horny. Umbonium. Isanda. Fam. 17.—Neritopsidae. Shell semi-globular, with short spire; operculum calcareous, not spiral. Neritopsis. Naticopsis, extinct. Fam. i8.—Macluritidae. Extinct, Cambrian and Silurian. Fam. 19.—Neritidae. Shell with very low spire, without umbilicus, internal partitions frequently absorbed; , a single ctenidium ; a cephalic penis present. Nerita, marine. Neritina, freshwater, British. Septaria, shell boat-shaped. Fam. 2o.—Titiscaniidae. Without shell and operculum, but with pallial cavity and ctenidium. Titiscania, Pacific. Fam. 21.—Helicinidae. No ctenidium, but a pulmonary cavity; heart with a single auricle, not traversed by the rectum. Hellcina. Eutrochatella. Stoastoma. Bourceria. Fam. 22.—Hydrocenidae. No ctenidium, but a pulmonary cavity; operculum with an apophysis. Hydrocena, Dalmatia. Fam. 23.—Proserpinidae. No operculum. Proserpina, Central America. Order 2.• PECTINIBRANCHIA.—In this order there is no longer any trace of bilateral symmetry in the circulatory, respiratory and excretory organs, the topographically right half of the pallial complex having completely disappeared, except the right kidney, which is T, Cephalic tentacle. br, One of the two symmetrical gills placed on'the neck. represented by the genital duct. There is usually a penis in the male. I movement. The " introvert " in these Gastropods is not the pharynx The ctenidium is monopectinate and attached to the mantle along as in the Chaetopod worms, but a prae-oral structure, its apical limit being formed by the true lips and jaws, whilst the apical limit of the Chaetopod's introvert is formed by the jaws placed at the junction of pharynx and oesophagus, so that the Chaetopod's introvert is part of the stomodaeum or fore-gut, whilst that of the Gastropod is external to the alimentary canal altogether, being in front of the mouth, not behind it, as is the Chaetopod's. Further, the Gastronod's introvert is pleurembolic (and therefore acrecbolic), and is limited both in eversion and in introversion; it cannot be completely everted owing to the muscular bands (fig. 19, G), nor can it be fully introverted owing to the bands (fig. 19, F) which tie the axial pharynx to the adjacent wall of the apical part of the introvert. As in all such intro- and e-versible organs, eversion of the Gastropod proboscis is effected by pressure communicated by the muscular body-wall to the liquid contents (blood) of the body-space, accompanied by the relaxation of the muscles which directly pull upon either the sides or the apex of the is effected directly by the contraction of these a, Siphon. d, The foot, expanded as in crawling. muscles. In various members of the Pectini- b, Head-tentacles. h, The mantle-skirt reflected over the sides branchia the mouth-bearing cylinder is in- C, Head, the letter placed near the right eye. of the shell. troversible (i.e. is a proboscis)—with rare exceptions these forms have a siphonate its whole length, except in Adeorbis and Valvata; in the latter alone it is bipectinate. There is a single well-developed, often pectinated osphradium. The eye is always a closed vesicle, and the internal cornea is extensive. In the radula there is a single central tooth or none. The former classification into Holochlamyda, Pneumochlamyda and Siphonochlamyda has been abandoned, as it was founded on adaptive characters not always indicative of true affinities. The order is now divided into two sub-orders: the Taenioglossa, in which there are three teeth on each side of the median tooth of the radula, and the Stenoglossa, in which there is only one tooth on each side of the median tooth. In the latter a pallial siphon, a well-developed proboscis and an unpaired oesophageal gland are always present, in the former they are usually absent. The siphon is an incompletely tubular outgrowth of the mantle margin on the left side, contained in a corresponding outgrowth of the edge of the shell-mouth, and serving to conduct water to the respiratory cavity. The condition usually spoken of as a " proboscis appears to be derived from the condition of a simple rostrum (having the mouth at its extremity) by the process-of incomplete introversion of that simple rostrum. There is no reason in the actual significance of the word why the term " proboscis " should be applied to an alternately introversible and eversible tube, connected with an animal's body, and yet such is a very customary use of the term. The introversible tube may be completely closed, as in the " proboscis " of Nemertine worms, or it may have a passage in it leading into a non-eversible oesophagus, as in the present case, and in the case of the eversible pharynx of the predatory Chaetopod worms. The diagrams here introduced (fig. 19) are intended to show certain important distinctions which obtain amongst the various " introverts," or intro-and e-versible tubes so frequently met with in animal bodies. Supposing the tube to be completely introverted and to commence its eversion, we then find that eversion may take ,place, either by a forward movement of the side of the tube near its attached base, as in the proboscis of the Nemertine worms, the pharynx of Chaetopods and the eye-tentacle of Gastropods, or by a forward movement of the inverted apex of the tube, as in the proboscis of the Rhabdocoel Planarians, and in that of Gastropods here under consideration. The former case we call "pleurecbolic " (fig. 19, A, B, C, H, I, K), the latter " acrecbolic " tubes or introverts (fig. 19, D, E, F, G). It is clear that, if we start from the condition of full eversion of the tube and watch the process of introversion, we shall find that the pleurecbolic variety is introverted by the apex of the tube sinking inwards; it may be called acrembolic, whilst conversely the acrecbolic tubes are pleurembolic. Further, it is obvious enough that the process either of introversion or of eversion of the tube may be arrested at any point, by the development of fibres connecting the wall of the introverted tube with the wall of the body, or with an axial structure such as the oesophagus; on the other hand, the range of movement of the tubular introvert may be unlimited or complete. The acrembolic proboscis or frontal introvert of the Nemertine worms has a complete range. So has the acrembolic pharynx of Chaetopods, if we consider the organ as terminating at that point where the jaws are placed and the oesophagus commences. So too the acrembolic eye-tentacle of the snail has a complete range of movement, and also the pleurembolic proboscis of the Rhabdocoel prostoma. The introverted rostrum of the Pectinibranch Gastropods presents in contrast to these a limited range of d" A, Simple introvert completely introverted. B, The same, partially everted by eversion of the sides, as in the Nemertine proboscis and Gastropod eye-tentacle = pleurecbolic. C, The same, fully everted. D, E, A similar simple introvert in course of eversion by the for-ward movement, not of its sides, but of its apex, as in the proboscidean Rhabdocoels=acrecbolic. F, Acrecbolic (=pleurembolic) introvert, formed by the snout of the proboscidiferous Gastropod. al, alimentary canal; d, the true mouth. ' The introvert is not a simple one with complete range both' in eversion and introversion, but is arrested in introversion by the fibrous bands at c, and similarly in eversion by the fibrous bands at b. G, The acrecbolic snout of a proboscidiferous Gastropod, arrested short of complete eversion by the fibrous band b. H, The acrembolic ( = pleurecbolic) pharynx of a Chaetopod fully introverted. at, alimentary canal; at d, the jaws; at a, the mouth; therefore a to d is stomodaeum, whereas in the Gastropod (F) a to d is inverted body-surface. I, Partial eversion of H. K, Complete eversion of H. mantle-skirt. On the other hand, many which have a siphonate mantle-skirt are not provided with an introversible mouth-bearing d D F a b al- CC c b O d C a al- d c H a rid c 6 r al- c -c c c- cylinder, but have a simple non-introversible rostrum, as it has been termed, which is also the condition presented by the mouth-bearing region in nearly all other Gastropoda. One of the best examples of the introversible mouth-cylinder or proboscis which can be found is that of the common whelk (Buccinum undatum) and its immediate allies. In fig. 23 the proboscis is seen in an everted state; it is only so carried when feeding, being withdrawn when the animal is at rest. Probably its use is to enable a, Anus. i, Intestine. r, Nephridium (kidney). r', Aperture of the nephridium. c, Heart. br, Ctenidium (gill-plume). pbr, Parabranchia (= the osphradium or olfactory patch). x, Glandular lamellae of the inner face of the mantle-skirt. y, Adrectal (purpuriparous) gland. t, Testis. vd, Vas deferens. p, Penis. mc, Columella muscle (muscular process grasping the shell). v, Stomach. h, Liver. N.B.—Note the simple snout or rostrum not introverted as a " proboscis." - the animal to introduce its rasping and licking apparatus into very narrow apertures for the purposes of feeding, e.g. into a small hole bored in the shell of another mollusc. The very large assemblage of forms coming under this order comprises the most highly developed predaceous sea-snails, numerous vegetarian species, a considerable number of freshwater and some terrestrial forms. The partial dissection of a male specimen of the common periwinkle, Littorina littoralis, drawn in fig. 2o, will. serve to exhibit the disposition of viscera which prevails in the group. The branchial chamber formed by the mantle-skirt overhanging the head has been exposed by cutting along a line extending back-ward from the letters vd to the base of the columella muscle mc, and the whole roof of the chamber thus detached from the right side of the animal's neck has been thrown over to the left, showing the organs which lie upon the roof. No opening into the body-cavity has been made; the organs which lie in the coiled visceral hump show through its transparent walls. The head is seen in front resting on the foot and carrying a median non-retractile snout or rostrum, and a pair of cephalic tentacles at the base of each of which is an eye. In many Gastropoda the eyes are not thus sessile but raised upon special eye-tentacles (figs. 25, 56). To the right of the head is seen. the muscular penis p, close to the termination of the vas deferens (spermatic duct) vd. The testis t occupies a median position in the coiled visceral mass. Behind the penis on the same side is the hook-like columella muscle, a development of the retractor muscle of the foot, which clings to the spiral column or columella ofthe shell (see fig. 33). This columella muscle is the same thing as the muscles adhering to the shell in Patella, and the posterior adductor of Lamellibranchs. The surface of the neck is covered by integument forming the floor of the branchial cavity. It has not been cut into. Of the organs lying on the reflected mantle-skirt, that which in the natural state lay nearest to the vas deferens on the right side of the median line of the roof of the branchial chamber is the rectum i', ending in the anus a. It can be traced back to the intestine i near the surface of the visceral hump, and it is found that the apex of the coil formed by the hump is occupied by the liver h and the stomach v. Pharynx and oesophagus are concealed in the head. The enlarged glandular structure of the walls of the rectum is frequent in the Pectinibranchia, as is also though not universal the gland marked y, next to the rectum. It is the adrectal gland, and in the genera Murex and Purpura secretes a colourless liquid which turns purple upon exposure to the atmosphere, and was used by the ancients as a dye. Near this and less advanced into the branchial chamber is the single renal organ or nephridium r with its opening to the exterior r'. Internally this glandular sac presents a second slit or aperture which leads into the pericardium (as is now found to be the case in all Mollusca). The heart c lying in the pericardium is seen in close proximity to the renal organ, and consists of a single auricle receiving blood from the gill, and of a single ventricle which pumps it through the body by an anterior and posterior aorta. The surface x of the mantle between the rectum and the gill-plume is thrown into folds which in many sea-snails (whelks or Buccinidae, &c.) are very strongly developed. The whole of this surface appears to be active in the secretion of a mucous-like substance. The single gill-plume br lies to the left of the median line in natural position. It corresponds to the right of the two primitive ctenidia in the untwisted archaic condition of the molluscan body, and does not project freely into the branchial cavity, but its axis is attached (by concrescence) to the mantle-skirt (roof of the branchial chamber). It is rare for the gill-plume of a Pectinibranch Gastropod to stand out freely as a plume, but occasionally this more archaic condition is exhibited as in Valvata (fig. 30). Next beyond (to the left of) the gill-plume we find the so-called parabranchia, which is here simple, but some-times lamellated as in Purpura (fig. 22). This organ has, without reason, been supposed to represent the second ctenidium of the typical mollusc, which it cannot do on account of its position. It should be to the right of the anus were this the case. Spengel showed that the parabranchia of Gastropods is the typical olfactory organ or osphradium in a highly developed condition. The minute structure of the epithelium which clothes it, as well as the origin of the nerve which is distributed to the parabranchia, proves it to be the same organ which is found universally in molluscs at the base of each gill-plume, and tests the indrawn current of water by the sense of smell. The nerve to this organ is given off from the superior (original right, see fig. 3) visceral ganglion. The figures which are given here of various Pectinibranchia are in most cases sufficiently explained by the references attached to them. As an excellent general type of the nervous system, attention may be directed to that of Paludina drawn in fig. 21. On the whole the ganglia are strongly individualized in the FIG. 22.—Female of Pectinibranchia, nerve-cell tissue being Purpura lapillus removed concentrated in the ganglia and absent from its shell ; the mantle-from the cords. At the same time, the skirt cut along its junction of the visceral loop above the left line of attachment intestine prevents in all Streptoneura the and thrown over to the shortening of the visceral loop, and it is right side of the animal rare to find a fusion of the visceral so as to expose the organs ganglia with either pleural, pedal or on its inner face. cerebral—a fusion which can and does a, Anus. take place where the visceral loop is not vg, Vagina. above but below the intestine, e.g. in gp, Adrectal purpuri- the Euthyneura (fig. 48), Cephalopoda parous gland. and Lamellibranchia. As contrasted r', Aperture of the nephwith the Aspidobranchia, we find that in radium (kidney). the Pectinibranchia the pedal nerves are br, Ctenidium (branchial distinctly nerves given off from the pedal plume). ganglia, rather than cord-like nerve- br', Parabranchia (=the tracts containing both nerve-cells or comb-like osphra- ganglionic elements and nerve-fibres. dium or olfactory Yet t in some Pectinibranchia (Paludina) organ). a ladder-like arrangement of the two pedal nerves and their lateral branches has been detected. The histology of the nervous system of Mollusca has yet to be seriously inquired into. The alimentary canal of the Pectinibranchia presents little diversity of character, except in so far as the buccal region is concerned. Salivary glands are present, and in some carnivorous forms (Dolium) these secrete free sulphuric acid (as much as 2 % is present in the secretion), which assists the animal in boring holes by means of its B, Buccal(suboeso- phageal) gan- glion. C, Cerebral ganglion. Co, Pleural ganglion. P, Pedal ganglion with otocyst at- tached. p, Pedal nerve. A, Abdominal ganglion at the extremity of the twisted visceral ' loop. sp, Supra-intestinal visceral ganglion on the course of the right visceral cord. sb, S u b - intestinal ganglion on the coursl of the left visceral cord. rasping tongue through the shells of other molluscs upon which it preys. A crop-like dilatation of the gut and a recurved intestine, embedded in the compact yellowish-brown liver, the ducts of which open into it, form the rest of the digestive tract and occupy a large bulk of the visceral hump. The buccal region presents a pair of shelly jaws placed laterally upon the lips, and a wide range of variation in the form of the denticles of the lingual ribbon or radula. Well-developed glandular invaginations occur in different positions on the foot in Pectinibranchia. The most important of these opens by the ventral pedal pore, situated in the median line in the anterior half of the foot. This organ is probably homologous with the:byssogenous gland of Lamellibranchs. The aperture, which was formerly supposed to be an aquiferous pore, leads into an extensive and often ramified cavity surrounded by glandular tubules. The gland has been found in both sub-orders of the Pectinibranchia, in Cyclostoma and Cypraea among the Taenioglossa, in Hemifusus, Cassis, Nassa, Murex, Fasciolariidae, Turbinellidae, Olividae, Marginellidae and Conidae among the Stenoglossa. It was discovered by J. T. Cunning-ham that in Buccinum the egg-capsules are formed by this, pedal gland and not by any accessory organ of the generative system. Such horny egg-capsules doubtless have the same origin in all other species in which they occur, e.g. Fusus, Pyrula, Purpura, Murex, Nassa, Trophon, Voluta, &c. The float of the pelagic Janthina, to which the egg-capsules are attached, probably is also formed by the secretion of the pedal gland. Other glands opening on or near the foot are: (I) The supra-pedal gland opening in the middle line between the snout and the anterior border of the foot. It is most commonly found in sessileshell-sac or shell-gland is well marked at this stage, and the pharynx is seen as a new ingrowth (the stomodaeum), about to fuse with and open into the primitively invaginated arch-enteron (fig. 26, F). In other Pectinibranchia (and such variations are representative for all Mollusca, and not characteristic only of Pectinibranchia) we find that there is a very unequal division of the egg-cell at the commencement of embryonic development, as in Nassa. Consequently e a, Siphonal notch of the shell e, Everted buccal introvert (pro-occupied by the siphonal boscis). fold of the mantle-skirt f, Foot. (Siphonochlamyda). g, Operculum. b, Edge of the mantle-skirt rest- h, Penis. ing on the shell. i, Under surface of the mantle- c, Cephalic eye. skirt forming the roof of the d, Cephalic tentacle. sub-pallial chamber. B, Sole of the foot of Pyrula tuba, to show a, the pore usually said to be " aquiferous " but probably the orifice of a gland; b, median line of foot. forms and in terrestrial genera such as Cyclostoma; (2) the anterior pedal gland opening into the anterior groove of the foot, generally present in aquatic species; (3) dorsal posterior mucous glands in certain Cyclostomatidae. The foot of the Pectinibranchia, unlike the simple muscular disk of the Isopleura and Aspidobranchia, is very often divided into lobes, a fore, middle and hind lobe (pro-, meso- and meta-podium, see figs. 24 and 25). Very usually, but not universally, the'metapodiuin carries an operculum. The division of the foot into lobes is a simple case of that much greater elaboration or breaking up into processes and regions which it undergoes in the class Cephalopoda. Even among some Gastropoda (viz. the Opisthobranchia) we find the lobation of the foot still further carried out by the development of lateral lobes, the parapodia, whilst there are many Pectinibranchia, on the other hand, in which the foot has a simple oblong form without any trace of lobes. The development of the Pectinibranchia has been followed in several examples, e.g. Paludina, Purpura, Nassa, Vermetus, Neritina. As in other Molluscan groups, we find a wide variation in the early process of the formation of the first embryonic cells, and their arrangement as a diblastula, dependent on the greater or less amount of food-yolk which is present in the egg-cell when it commences its embryonic changes. In fig. 26 the early stages of Paludina vinipara are represented. There is but very little food-material in the egg of this Pectinibranch, and consequently the diblastula forms by invagination; the blastopore or orifice of invagination coincides with the anus, and never closes entirely. A well-marked trochosphere is formed by the development of an equatorial ciliated band ; and subsequently, by the disproportionate growth of the lower hemisphere, the trochosphere becomes a veliger. The primitive
End of Article: GASTROPODA
GASTRITIS (Gr. yarn*, stomach)

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