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PHORA

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Originally appearing in Volume V06, Page 251 of the 1911 Encyclopedia Britannica.
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PHORA, Pilsbry. — Insertion plates well developed and Z slit. Fam. 2. Ischnochitonidae. —All the valves with slits, and the inner layer well covered by the .touter. Subfam. 1. Ischnochitoninae. — No shell=eyes : sutural laminae separated; slits in the valves 1-7 do not correspond with the ribs of the tegmentum. Ishcnochiton, Trachydermon, Chaetopleura, Stenoplax, Stenoradsia. P From Gegenbaur, Elements of Comp. Anatomy. B, Buccal ganglia (concerned with the odontophore). C, Cerebral nerve-mass. P, Pedal ganglion and com- mencement of pedal nerve-cord. pl, Visceral nerve-cord. The sublingual ganglia are not lettered. Subfam. 2. Callochitoninae. With shell-eyes and united sutural laminae. Callochiton laevis, North Atlantic and Mediterranean. Subfam. 3. Callistoplacinae. No shell-eyes, slits in the valves 1-7 corresponding with the ribs of the tegmentum. Callistochiton (viviparous). Nuttalochiton. Fam. 3. Mopaliidae. Each intermediate valve with a single slit; girdle hairy. Mopalia, Placiphorella, Plaxiphora, Placophoropsis. Fam. 4. Acanthochitonidae. Valves immersed in the girdle, with small tegmentum. Acanthochiton (A. fascicularis, North Atlantic and Mediterranean. Spongiochiton, Katharina, Amicula, Cryptochiton (C. stelleri, arctic). Fam. 5. Cryptoplacidae. Vermiform, with thick girdle and small valves; insertion and sutural plates strongly drawn forward, sharp and smooth. Cry ptoplax, Choneplax. Suborder III. TELEOPLACOPHORA, Pilsbry.—All the valves, or atleast the seven anterior, with insertion plates cut into teeth by slits. Fam. 6. Chitonidee. Characters of the suborder. Subfam. 1. Chitoninae. No extra-pigmental eyes; insertion plates with pectinations between the fissures. Chiton, Eudoxochilon, Trachyodon, Redsia. Subfam. 2. Toniciinae. Extra-pigmental shell-eyes. Tonicia, Acanthopleura, Enoplochiton, Onithochiton, Schizochiton, Lorica, Loricella, Liolophura. Order 2.—APLACOPHORA, von Jhering. Chaetoderma was first described by S. Lovell, in 1841, and was for a long time believed to be a Gephyrean worm. Neomenia, mentioned first by Michael Sars in 1868 under the name Solenopus, was afterwards included among the Opisthobranchs by J. Koren and D. C. Danielssen. C. Gegenbaur placed the two genera in a division of Vermes which he called Solenogastres. The chief points in which the Aplacophora differ from the Polypiacophora are: (I) they are worm-like in shape; (2) there is no distinct foot, and the mantle bears no shell-valves, but only numerous calcareous spicules; (3) the digestive tube is straight. Neomenia and its allies are marine animals living at depths of 15 to 800 fathoms on soft muddy ground; they are found crawling on corals and hydrozoa, on which they feed. The British genera are: Ncomenia, Rhopalomenia and Myzomenia. They have been taken in nearly all seas except the South Atlantic and S.E. and N.W. Pacific. About forty species are known. Chaetoderma, of which nine species have been described, has similar habits and distribution, but feeds chiefly on Protozoa. The order Aplacophora is divided into two suborders. Suborder I. NEOMENIOMORPHA.—Aplacophora with a distinct longitudinal ventral groove; bisexual with paired genital glands and no distinct liver. The whole of the skin except the ventral groove corresponds to the mantle of Chiton. The cuticle, in some species very thick, contains numerous spicules which are long, hollow and calcified; they are secreted by epithelial papillae. In some species there are also sensory papillae comparable to the aesthetes of Chitons. A small longitudinal projection in the ventral groove represents the A B C D Fin. 9.—Neomenia carinata, Tullberg (after Tullberg). A, Lateral view. a, Anterior. B, Ventral view. b, Posterior extremity. C, Dorsal view. c, Furrow, in which the narrow D, Ventral view of a more ex- foot is concealed. tended specimen. foot. Into the groove open mucous glands, a large one anteriorly and another opening into a posteriorly cloacal, branchial cavity. Branchiae.—In Neomeniidae and most of the Parameniidae there is a circlet of gills on the inner walls of the cloacal chamber. These gills are simple folds or laminae of the body wall. In other species they are absent. Intestine.—The mouth opens into a muscular pharynx lined by a thick cuticle. Into the pharyngeal cavity open salivary glands and radular sac. The former are paired and ventral, and open on a subradular prominence. In some species there is a second dorsal pair. Neomenia and other genera have no salivary glands. The radula when present comprises several transverse rows of teeth, and each transverse row may have several teeth (polystichous), two teeth (distichous), or one tooth (monostichous). It is a curious fact that in the original type Neomenia the radula is entirely absent, as it likewise is in several genera of Proneomeniidae. The oesophagus is short and leads into a long, straight stomach, provided with numerous symmetrical lateral caeca. The stomach opens into a short straight rectum which opens into the branchial chamber. Coelom, Gonads and Excretory Organs.—The coelom differs from that of the Chitons in the fact that the cavities of the genital organs are continuous with it, and in the fact that there is only one pair of coelomoducts resembling the renal organs of Chitons, but serving also as genital ducts. The gonads are paired and hermaphrodite, they form a pair of anterior prolongations of the pericardium, extending nearly to the anterior end of the body. Ova are developed on the median, spermatozoa on the outer wall of each genital tube. The pericardium is ciliated internally on its dorsal and lateral walls. The urino-genital tubes arise from the posterior angles of the pericardium, pass first forwards, then backwards, and unite to open by a common opening into the cloaca below the anus except in PC After Hubrecht, loc. cit. A, Proneomenia. B, Neomenia. C, Chaetoderma. D, Chiton. c, Cerebral ganglia. s, Sublingual ganglia. v, Pedal (ventral) nerve-cord. 1, Visceral (lateral) nerve-cord. pc, Post-anal junction of the visceral nerve-cords. A PC Strophotnenia, where the openings are separate. Usually each tube is provided with caecal appendages on its proximal portion, and these serve as vesiculae seminales, while the distal portion is enlarged and glandular and secretes the egg-sheII. Heart and Vascular System.—There is a heart in the pericardium consisting of a median ventricle attached, except in Neomenia, to the dorsal wall of the pericardium, and in Neomenia a pair of auricular ducts returning blood from the gills to the ventricle. The aorta is not independent as in Chitons, but is a sinus Iike the other channels of the circulation. A single median ventral sinus passes backwards to the gills or cloaca. The blood is coloured red by haemoglobin in blood corpuscles. Nervous System.—Ganglionic enlargements are more conspicuous than in the Chitons. In front of the buccal mass is a median cerebral ganglion. From this pass off two pairs of cords, the pleural and pedal, in Proneomenia separate from their origin, in Neomenia united at first and diverging at a pleural ganglion. The pedal cords anteriorly form a pair of pedal ganglia united by a thick commissure. The supra-rectal commissure may be present and bear an ovoid ganglion; or may be wanting. With regard to sense organs the epithelial papillae of the mantle have been mentioned. There is also in some genera a median retractile sensory papilla on the dorsal posterior surface above the rectum, not covered by the cuticle. Development has only been described in Myzomenia banyulensis, by G. Pruvot. It closely resembles in the early stages that of Chitons. The external surface of the trochosphere is formed of a number of ciliated test-cells. The ectoderm behind the ciliated ring develops spicules, and the post-oral region of the larva elongates. Later the ciliated ring or velum disappears and seven imbricated calcareous plates, made up of flattened spicules, are formed on the dorsal surface. This appears to indicate that the Neomeniomorpha are descended from Chiton-like ancestors, and that they have lost their shell valves. Classification of the NEOMENIOMORPHA.—Fam. I. Lepidomeniidae. Slender, tapering behind, with subventral cloacal orifice; thin cuticle without papillae; flattened spicules; no gills. Lepidomenia, Ismenia, Ichthyodes, Stylomenia, Dondersia, Nematomenia, Myzomenia, M. banyulensis, Mediterranean and Plymouth. Fam. 2. Neomeniidae. Short, truncate in front and behind; cloacal orifice transverse; gills present; rather thin cuticle; no radula. Neomenia (N. carinata, N. Atlantic and N. and N.W. Scotland), Hemimenia. Fam. 3. Proneomeniidae. Elongated, cylindrical, rounded at both ends; thick cuticle with acicular spicules; radula polystichous or wanting. Proneomenia, Amphimenia, Echinomenia, Rhopalomenia (R. aglaopheniae, Mediterranean and Plymouth), Notomenia, Pruvotia, Strophomenia. Fam. 4. Parameniidae. Short and truncated in front; thick cuticle, often without papillae; gills and ` radula present. Paramenia, Macellomenia, Pararhopalia, Dinomenia, Cyclomenia, Proparamenia, Uncimenia, Kruppomenia. Suborder II. CHAETODERMOMORPIIA.—Aplacophora without distinct ventral groove, with single median unisexual gonad, with differentiated hepatic sac, and with cloacal chamber furnished with two bipec- tinate gills. There are only two genera• in this sub- order: Chaetoderma, and terior; the posterior ex- e tremity forms the enlarged funnel-like branchial or cloacal chamber. The anterior extremity is also somewhat enlarged. The whole surface is uniformly covered with short compressed calcareous spicula embedded in the cuticle. Branchiae.—The single pair of branchiae are placed symmetrically right and left of the anus, and each has the structure of a ctenidium bearing a row of lamellae on each side as in the Polyplacophora. Intestine.—The mouth is anterior, terminal and crescentic, and beneath it is a rounded ventral shield. On the floor of the pharynx or buccal mass is a rudimentary radula, which in many species consists of a single large tooth, bearing two small teeth or a row of teeth. In other species the radula is more of the usual type consisting of several transverse rows of two or three teeth each. Two pairs of salivary glands open into the buccal cavity. The digestive tube is straight and simple, wider in its anterior part, into which opens the duct of the hepatic caecum (fig. 4, B). The latter extends backwards on the ventral side of the intestine. Coelom, Gonads and Excretory Organs.—These are closely similar in their relations to those of the Neomeniomorpha. The chief difference is that the gonad or generative portion of the coelom is single and median, opening into the pericardium by a single posterior aperture. The excretory organs or coelomoducts arise from the posterior corners of the pericardium, run forwards and then back-251 wards to open by separate apertures lateral to the gills (fig. 5, A). There are no accessory generative organs. The heart and vascular system are similar to those of the Neomeniomorpha, the only important differences being that the ventricle is nearly free in the pericardial cavity, and that the latter is traversed by the retractor muscles of the gills. Nervous System.—T here are two closely connected cerebral ganglia, from which arise the usual two pairs of nerve cords. Pallial and pedal on each side are closer together than in the other groups, and posteriorly they unite into a supra-rectal cord provided with a median ganglionic enlargement (fig. 7, C). A small stomatogastric commissure bearing two small ganglia arises from the cerebral ganglia and surrounds the oesophagus. The development is at present entirely unknown. General Remarks on the Amphineura. The most important theoretical question concerning the Amphineura is how far do they represent the original condition of the ancestral mollusc? That is to say, we have to inquire which of their structural features is primitive and which modified. Their bilateral symmetry is obviously to be regarded as primitive, and the nervous system shows an original condition from which that of the asymmetrical twisted Gastropods can be derived. But in many other features both external and internal the three principal divisions differ so much from one another that we have to consider in the case of each organ-system which condition is the more primitive. According to Paul Pelseneer the Polyplacophora are the most archaic, the Aplacophora being specialized in (r) the great reduction of the foot, (2) the disappearance of the shell (Cryptoplax among the Polyplacophora showing both reductions in progress), (3) the disappearance of the radula. But it is a widely recognized principle of morphology that a much modified animal is by no means modified to the same degree in all its organs. A form which is primitive on the whole may show a more advanced stage of evolution in some particular system of organs than another animal which is on the whole more highly developed and specialized. Thus the independent metamerism of certain organs in the Chitons is not primitive but acquired within the group: e.g. the shell valves and the ctenidia. And although embryology seems to prove that the Neomeniomorphs are derived from forms with a series of shell-valves, nevertheless it seems probable that the calcareous spicules which alone are present in adult Aplacophora preceded the solid shell in evolution. It is held by some morphologists that the mollusc body is unsegmented, and therefore is to be compared to a single segment of a Chaetopod or Arthropod. In this case there should be only one pair of coelomoducts in the adult, the pair of true nephridia which should also occur being represented by the larval nephridia. There should also be only a single coelom, or a pair of lateral coelomic cavities. On this view then the Aplacophora are more primitive than the Polyplacophora in the relations of coelom, gonad and coelomoducts; and the genital ducts of the Chitons have arisen either by metameric repetition within the group, or by the gradual loss of an original connexion between the generative sac and the renal tube, as in Lamellibranchs and Gastropods, the generative sac acquiring a separate duct and opening to the exterior on each side. Proneomenia Sluiteri," Nied. Arch. f. Zool. Suppl. I., 1881; A. Kowalewsky and A. F. Marion, " Contr. A l'histoire des Solenogastres ou Aplaeophores," Ann. Mus. Marseille, Zool. iii., 1887; A. Kowalewsky, " Sur le genre Chaetoderma," Arch. de zool. exper. (3) ix., 1901; P. Pelseneer, " Mollusca," Treatise on Zoology, edited by E. Ray Lankester, pt. v., 1906; E. Ray Lankester, " Mollusca," in the 9th ed. of this Encyclopaedia, to which this article is much indebted. (J. T. C.)
End of Article: PHORA
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