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SELACHIANS, or ELASMOBRANCHII

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Originally appearing in Volume V24, Page 597 of the 1911 Encyclopedia Britannica.
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SELACHIANS, or ELASMOBRANCHII, a subclass of fishes, including the various kinds of Sharks and Rays. Structural Features.—The general shape is somewhat spindle-like in the Sharks, while in the Rays—in correlation with the ground-feeding habits—the body has become greatly depressed. Departures from the normal are seen in the Hammerheads (Sphyrna), where the sides of the head are so produced as to give a hammer shape, and in the Saw-fishes (Pristis), where the head is prolonged forwards as a greatly elongated flattened rostrum. In regard to the fins, the tail is heterocercal in the adults of living forms, except in Chlamydoselachus, where the protocercal condition persists; the pectoral fins are greatly enlarged in the Rays, in which movement is effected mainly by the passage backwards of waves of flexure along the pectoral fins; the pelvic fins in the last-named fishes have their hinder portions modified in the male to form special copulatory organs, the myxipterygia or " claspers." The mouth opening is a ventrally placed crescentic slit except in Chlamydoselachus, where it is nearly terminal. The olfactory organs, lying in front of the mouth, are widely open to the exterior, and in some cases are connected with the mouth by oronasal grooves. The spiracular opening frequently retains in the adult an opening to the exterior behind or below the eye. In the Rays it is used mainly for inspiration. The post-spiracular clefts open freely to the exterior, each guarded by a flap-like extension of its anterior margin which serves as a valve to allow water to pass only in one direction, viz. outwards. In the Holocephali the anterior flap, that arising from the hyoid arch, is greatly enlarged so as to form anoperculumcovering over all the clefts lying posterior to it. The postspiracular clefts are usually five in number, but six in Chlamydoselachus and Notidanus griseus, and seven in N. cinereus. The gill lamellae are strap-like and attached by their edges to the gill septa. Fully developed lamellae are present on the anterior wall of the hyobranchial clefts and vestigial lamellae on the anterior wall of the spiracle where they form the " pseudobranch." In the Basking Shark Cetorhinus the pharyngeal openings of the gill clefts are guarded by series of long slender rods—the greatly elongated representatives of the small conical " gill rakers " found in this position in other fishes. These structures form a sieve-like arrangement for preventing the minute creatures (plankton) upon which this shark feeds from passing out through the gill clefts. There appears to be no representative of the lung or swimbladder, and there are no pyloric caeca. The intestine is provided with a spiral valve in its interior which varies in character in different forms (I) A glandular caecum—the rectal caecum—opens into the dorsal side of the rectum. In regard to the coelomic spaces the Selachians exhibit the interesting feature that the pericardiac cavity is in the adult in communication with the general splanchnocoele by an open channel sometimes forked at its posterior end. This communication apparently arises secondarily and is not due to a persistence of the embryonic communication (2). In the case of Torpedo and in the ordinary Rays certain portions of the muscular system are converted into electrical organs. In the Skates and Rays the electrical disturbance is relatively small—imperceptible by human beings—but in Torpedo it is very considerable. No doubt the electric organs subserve a defensive function. The kidney of the adult is a mesonephros. The pronephros is never functional, though it appears in a vestigial form in the embryo. The mesonephros shows a division into a broader posterior portion which alone is renal in function, and a slender anterior portion which in the male subserves a genital function. The female genital duct is a typical Mullerian duct having at its anterior end a wide coelomic funnel and lined by glandular epithelium whose secretion forms adventitious coats round the egg during its downward passage. The spermatozoa find their way to the cloaca by way of the mesonephric duct, the hinder portion of which is dilated to form a vesicula seminalis. The urino-genital sinus—formed by the fusion of the mesonephric ducts at their hinder ends—projects forward as a pair of pockets (the so-called sperm sacs). The skeleton of the Selachian shows remarkably' archaic features, inasmuch as the internal skeleton is entirely cartilaginous, the bony or placoid skeleton retaining its primitive superficial position and not showing in any part a tendency to sink or spread inwards for the reinforcement of the cartilaginous skeleton. The vertebral column is of the chordacentrous type, although in some of the more archaic of known fossil forms (Pleuropterygii, Ichthyotomi, Acanthodei, Hybodus) the chondrified secondary sheath of the notochord apparently retained in the adult the unsegmented condition. The same holds for the Holocephali and for the hinder part of the vertebral column of the existing Chlamydoselachus. The centra are usually, if not always, strengthened in the adult by the deposition of lime salts in the intercellular matrix: such calcified cartilage must be carefully distinguished from true bone. The arrangement of the calcified tracts shows differences which are of taxonomic importance. In the cyclospondylous type (fig. r, A) the calcified tract has the form of a double cone—of the wall of a dice-box—and in the transverse section appears as a simple circle (Palaeo- spinax, Acanthias, Scymnus). In the d' tectospondylous (fig. d [) eCCnrk .d, }~q - .'dam r, B) type, ad * ditional calcified tracts are developed outside and concentric with the original double cone (Batoidei), while in the asterospondylous (fig. i, ) type the ad- ditional calcifica- tion takes the form of longitudinally arranged plates radiating outwards from the original double cone, so as to produce a star-like appearance in cross section (Scyllium, Lamna). Eventually in the adult the calcification may extend from the special tracts above mentioned throughout the whole centrum. In certain cases (Carchariidae, &c.) the transverse section of the centrum is modified by its surface becoming indented by the ingrowth of cartilage tracts (calcified or not) situated external to the primary sheath, thus producing an appearance something like a Maltese cross. The arch elements of the vertebral column have lost in variable degrees the numerical correspondence with the centra which they possibly once possessed. The same applies to the relations of the h.a, Haemal arch. n.a, Neural arch. A B C From Zittel's Handbuch der Pal&ontologie, by permission of Herren R. Oldenbourg, Publishers, Munich. d, d', d", Calcified tracts. ex.m, Primary sheath. centra with the fundamental body metamerism, as shown by the neuro-muscular segments; e.g. there are frequently in the caudal region in shark, (3) two centra to each neuro-muscular segment, while in part of the trunk in Notidanidae one centrum corresponds to two neuro-muscular segments. The chondrocranium retains through life its primitive character. The ethmoidal region is prolonged forwards into a rostrum—which may be of enormous size (Pristis), or may be of insignificant dimensions as in most sharks. The jaw apparatus is also remarkably archaic: the functional jaws being the palatopterygoquadrate cartilage and Meckel's cartilage respectively. The suspension from the skull is typically hyostylic, except in Notidanus where it is amphistylic, in the Holocephali where it is autostylic, and in Heterodontus where it approaches the autostylic condition. The skeleton of the postmandibular visceral arches consists of a half hoop of cartilage on each side divided into a number of segments: the two half hoops are connected ventrally by a median copula (basihyal, or basibranchial). The hyoid arch most usually shows a division into a dorsal (hyomandibular) and a ventral (ceratohyal) element, and except in the Notidanidae the dorsal segment is of large size in correlation with its function in the suspension of the jaws. This enlargement of the hyomandibular is particularly marked in the case of the Rays (Raia) where it may become freed from the ventral segmented part of the arch which articulates directly with the skull. The branchial arches usually are segmented on each side into four pieces (pharyngobranchial, epibranchial, ceratobranchial and hypobranchial) in addition to the median copula. All these visceral arch skeletons bear on their outer surface a number of cartilaginous rays which radiate outwards and support the gill septa. Those attached to the hyoid arch (branchiostegal rays) show by their specially large size a foreshadowing of the development of the operculum of the higher group of fishes. In addition to the elements already mentioned slender cartilaginous rods of doubtful significance are found superficial to the jaw cartilage (labials) and to certain of the branchial arches (extra branchials). The limb girdles of the Selachians are very simple—a hoop of cartilage incomplete dorsally in the case of the pectoral, a transverse bar of cartilage in the case of the pelvic girdle. In the ancient Pleuracanthids the two halves of the pectoral girdle remained distinct in the adult, and each was segmented into three pieces, thus showing a remarkable correspondence with the visceral arches lying in front of them. (For the bearing of this on theories of the origin of limbs see ICHTHYOLOGY: Anatomy.) In some existing sharks (e.g. Acanthias) a relic of this condition is found—the dorsal extremity of the girdle being segmented off from the rest. The cartilaginous skeleton of the pectoral limb consists of numerous cartilaginous rays which typically are connected with the girdle through the intermediary of three basal pieces known as pro pterygium, mesopterygium and metapterygium. In the Rays, in correlation with the gigantic development of the pectoral fins, the propterygium and metapterygium become greatly enlarged in an anteroposterior direction—the former becoming attached to the side of the cranium or even meeting and fusing with its fellow in front (Trygon). In the pelvic limb the rays are—except a few in front—borne on the outer side of a single backwardly projecting basal piece (metepterygium). In the male this is continued backwards to form the skeleton of the clasper. The limb skeleton shows remarkably interesting features in the ancient extinct sharks Cladoselache and Pleuracanthus. The placoid or bony skeleton is seen in its most archaic form in Selachians in the form of superficially placed placoid scales. These may be uniform in size forming the characteristic shragreen of the various sharks, or scattered scales may be greatly enlarged as in the thornbacks, or finally the scales may have completely atrophied as in the electric ray (Torpedo). Local placoid elements or aggregations of placoid elements may become specially enlarged to form defensive or offensive weapons. In the sawfish (Pristis) a row of greatly enlarged placoid spines along each side of the rostrum form the " teeth ' of the saw, and a similar condition occurs in the sharks of the genus Pristiophorus. In the sting-rays the tail is armed with a large serrated spine taking the place of the dorsal fin and having behind it smaller spines, the front one of which increases in size and becomes functional if the previously functional spine is broken off. The portion of skin involuted to line the buccal cavity carries with it its armature of placoid scales (Chlamydoselachus). Normally these undergo atrophy except near the margin of the cavity where they are greatly enlarged to form the teeth. These vary greatly, as might be expected, in accordance with the nature of the food—they may be sharp prehensile spines, or triangular cutting blades with serrated edges (e.g. carcharodon and other sharks) or flattened plates adapted to crushing Molluscan shells (e.g. various rays). Vascular System.—The heart possesses a single atrium and a single ventricle. Opening into the atrium is a well-developed sinus venosus and leading from ventricle into ventral aorta is a well-developed rhythmically contractile conus arteriosus, containing a complex arrangement of pocket valves. These pocket valves are arranged in longitudinal rows, each row representing the remains of a longi-tudinal ridge in the conus of the embryo. The valves of, each row tend to become differentiated in size, e.g. in Rhina the anterior valve in each row is considerably enlarged. Finally a condition may be reached in which all the valves of the row disappear except two as in Scyllium canicula. As regards the remaining parts of the blood-vascular system, probably the most characteristic feature is the tendency seen in various Selachians for the main venous trunks (cardinals and hepatic veins) to become dilated at their front ends into a special sinus which fills the cavity of the orbit. The kidneys are provided with a well-developed renal portal system. Nervous System.—The brain of the Selachians shows a mixture of primitive and specialized characters. The hemisphere region is remarkable for the indistinctness of the two hemispheres. This has been looked on by some, e.g. Gegenbaur, as a primitive feature, the hemispheres having not yet been developed. To others, including the writer of this article, the balance of evidence seems in favour of the condition in Selachians being due to a secondary disappearance of the separation between the two hemispheres. In such comparatively primitive forms as the Notidanidae the paired character of the hemisphere region is still clearly indicated. In the Raiidae on the other hand even the lateral ventricles have lost their paired character, while in Myliobatis the ventricle of the region has disappeared entirely, leaving a solid unpaired mass. Although the hemisphere region has in great part lost its paired character, this does not apply to the anterior outgrowths from the hemispheres, the olfactory lobes. In the Holocephali the olfactory lobes remain close to the hemisphere surface. In other Selachians, however, the olfactory organ, with the olfactory lobe attached to it, becomes carried away by differential growth to a lesser or greater distance from the hemisphere. The result is that the middle part of the. olfactory lobe becomes greatly drawn out (Olfactory tract or peduncle). The swelling at its anterior end is now spoken of as the olfactory lobe, while its hinder end, where it passes into the brain, is the olfactory tubercle. In the region of the thalamencephalon there is a well-developed infundibular gland, and the pineal body is present in the form of a greatly elongated slender tube which passes upwards and forwards to end in contact with the cranial roof about the level of the anterior boundary of the hemisphere region. The pineal body ends in a small bulbous enlargement but shows no trace of eye structure. In the mesencephalon are a pair of well-developed optic lobes. The cerebellum is highly developed—as in the case of other fishes which perform active and complex movements. The medulla oblongata shows a characteristic feature in Torpedo, where the nucleus of origin of the electric nerves forms a large swelling on the floor of the fourth ventricle on each side of the mesial plane. In connexion with the organs of special sense in the Selachians, there are various points of general interest. In various forms, e.g. Scyllium and Raia, the olfactory organ is connected with the mouth by means of an open gutter—the oronasal groove—in which we may probably see the homologue of the similar groove which appears in the embryo of the higher vertebrates and which, becoming covered in, gives rise to the communication between nose and buccal cavity via the internal pares. The otocyst or auditory organ, which arises in ontogeny as an involution of the ectoderm, is remarkable in the Selachians from the fact that it does not become completely enclosed. Throughout life the ductus endolymphaticus remains open to the exterior by a minute pore on the dorsal side of the head. In Rhina (4) this communication of otocyst with exterior is relatively wide, and through it grains of sand gain admission to the interior of the otocyst, where they take the place functionally of the small calcareous otoconia of other forms. Cutaneous Sense Organs.—As in other fishes there is a rich development of sense buds scattered over the general surface of the head and body. Certain of these retain their superficial position through-out life, while others are carried inwards by involution of the ectoderm so that they come to be sunk in pits. These pits may become prolonged into tubes with dilatations at their inner ends containing the sense buds (" Ampullae of Lorenzini " of the head region), or their external. opening may be narrowed to a fine slit, or they may become completely shut off from the exterior (" Savi's vesicles " on ventral side in Torpedo). Another series of these cutaneous sense buds is arranged in rows on the head and trunk to form the characteristic organs of the lateral line. These are innervated by the lateralis system of nerves. These organs, like the sense buds already mentioned, become sunk beneath the surface, lying first in the floor of an open groove (Chimaera) and later, as this becomes covered in, in a canal which opens to the exterior at intervals by pores. Ontogenetic Development.—The Selachians possess large heavily yolked eggs and show corresponding modifications. in their develop-mental processes. Segmentation is partial, resulting in the formation of a blastoderm. The process of gastrulation is much less, modified than in the Sauropsida (where similar conditions prevail as regards quantity of yolk), and can be readily compared with the method seen in the larger types of holoblastic egg. Fertilization is internal, the myxipterygia or claspers serving as intromittent organs. On its passage down the oviduct the egg normally becomes surrounded by a layer of albumen and by a tough external envelope of flattened quadrangular shape. The corners of the external capsule may be produced into points (Raia) or into long 596 tendril-like structures (Scyllium) which serve to anchor it to sea-weeds. In a large number of Selachians the adoption of internal fertilization has been followed by the retention of the embryo within the oviduct (uterus) for a prolonged period. In such cases we find interesting adaptive arrangements for aiding the nutrition and respiration of the young individual. The highly vascular wall of the yolk sac may come into intimate relation with the uterine lining, so as to form a simple yolk sac placenta (Mustelus laevis, &c.). In other forms the uterine lining secretes a nutritive fluid or uterine milk which apparently is taken into the alimentary canal of the embryo through the spiracles (Myliobatis sp., Taeniura sp.). In certain Rays (Pteroplataea micrura) this secretory activity of the uterine lining is concentrated in long villous processes known as trophonemata, which pass through the wide spiracles of the young fish and pour their secretion directly into the cavity of its alimentary canal. CLASSIFICATION The following table gives a convenient classification (taken from Bridge (5)) of those Selachians at present known:—Order I. Pleuropterygii (Extinct : palaeozoic). I I. Acanthodii (Extinct: palaeozoic mainly). „ III. Ichthyotomi (Extinct: palaeozoic mainly). „ IV. Plagiostomi. Suborder I. Squali (Selachii s.s.). Fam. 1. Notidanidae (Notidanus = Hexanchus and Heptanchus). 2. Chlamydoselachidae (Chlamydoselachus). 3. Heterodontidae (Heterodontus = Cestracion). 4. Cochliodontidae (Extinct: palaeozoic). 5. Psammodontidae (Extinct : palaeozoic). 6. Petalodontidae (Extinct: mainly palaeozoic). 7. Scylliidae (Scyllium, Pristiurus, Stegostoma). 8. Carchariidae (Carcharias, Galeus, Galeocerdo, Muslelus). 9. Sphyrnidae (Sphyrna = Zygaena). to. Lamnidae (Lamna, Carcharodon,Alopecias,Mitsukurina). 11. Cetorhinidae (Cetorhinus). 12. Rhinodontidae (Rhinodon). 13. Spinacidae (Acanthias, Spinax, Scymnus, Laemargus, Echinorhinus). 14. Rhinidae (Rhina). 15. Pristiophoridae (Pristiophorus). Suborder II. Batoidei. Fam. I. Pristidae (Pristis). 2. Rhinobatidae (Rhinobatus). 3. Raiidae (Raia). „ 4. Tamiobatidae (Extinct: palaeozoic). 5. Torpedinidae (Torpedo: Narcine). „ 6. Trygonidae (Trygon, Pteroplataea, Taeniura). „ 7. Myliobatidae (Myliobatis, Aetobatis, Ceratoptera). Order V. Holocephali. Fam. 1. Ptychodontidae (Extinct: palaeozoic). 2. Squaloraiidae (Extinct: mesozoic). „ 3. Myriacanthidae (Extinct: mesozoic). „ 4. Chimaeridae (Chimaera, Callorhynchus, Harriotta). Existing Forms.—The Selachians known to survive to the present day are confined to orders IV. and V., the former including the Sharks (Squall) and Rays (Batoidei), and the latter including the remarkable Chimaera and its allies. For the more interesting members of the Plagiostomi see SHARK and RAY. The general morphological features of the Plagiostomi are dealt with in the article Ichthyology. It remains now to refer shortly to one or two of the subdivisions which contain forms of special morphological interest from their in many respects primitive character. Such families are the Notidanidae, the Chlamydoselachidae and the Heterodontidae. The second of these is of very special interest: it contains the single living genus Chlamydoselachus, specimens of which have been obtained in considerable numbers from deep water off the coast of Japan, while isolated specimens have been taken off the coasts of Australia and Norway and near Madeira. The general shape of Chlamydoselachus is elongated, almost eel-like (fig. 2). The mouth is nearly terminal, instead of being well back on the ventral surface as in other sharks. The teeth are very characteristic, flattened in shape, pointing backwards and over-lapping one another in longitudinal rows. Each tooth has three slender pointed cusps and closely resembles the teeth of various members of the extinct group Ichthyotomi. The small placoid elements which cover the general body surface are seen to become enlarged at the margin of the mouth, especially posteriorly, these enlarged placoid elements functioning as accessory teeth and in fact being practically teeth in an early stage of evolution. It is interesting to note also that the lining of the mouth still develops a covering of placoid elements. (In the typical gnathostome the placoid elements have of course disappeared from the mouth lining,except in the case of the functional teeth.) There is no oronasal groove in the adult, and the spiracle is greatly reduced. The valvular flaps guarding the external openings of the gill (6) clefts are much larger than in other sharks, particularly the most anterior (hyoidean) which meets its fellow ventrally and is prolonged backwards for some distance as an incipient operculum. The tail is practically protocereal, although the median fin-fold is considerably more developed on its ventral side than dorsally. The lateral line organs on the sides of the body are situated at the bottom of an open groove; only in the head region has this become covered in. The Notidanidae, Iike Chlamydoselachus, show more than the ordinary number of gill clefts. Notidanus griseus (Hexanchus) has six, while N. cinereus (Heptanchus) has seven postspiracular gill-clefts. In both Notidanidae and Chlamydoselachidae the vertebral column shows very primitive features with either very slight calcification or none at all. The Heterodontidae include the recent genus Heterodontus (= Cestracion), the Port Jackson shark or Bullhead shark, widely distributed through the Pacific. Numerous Mesozoic and possibly also Palaeozoic forms belong to this family. The small and nearly terminal mouth, the amphistylic skull, and the egg cases with an external spiral lamina are characteristic features. Palaeontological History (6).—It must be borne in mind that the sharply delimited groups into which animals appear to be divided are due to our imperfect knowledge, to the fact that our knowledge is limited to short isolated periods of geological time. Were our knowledge of palaeontology complete, it would be found that the various groups graded into one another by insensible gradations, so that it would be quite impossible to set definite limits to any one group. Already even in the extraordinarily imperfect condition of palaeontological knowledge this difficulty is making itself felt, and in the remains from the older deposits it becomes difficult to decide which of the recognized groups the various forms are most closely allied to. Amongst the most ancient forms of fishes known at present are the remarkable Ostracodermi of the Upper Silurian and Devonian. The general form of these creatures gives the impression that they were ground-feeding fishes which had become highly specialized along much the same lines as the rays amongst existing Selachians. In the highly interesting Coelolepidae described by Traquair (7) from the Upper Silurian and Devonian and comprising the genera Thelodus and Lanarkia a placoid skeleton is present, the individual elements being in the form of small hollow spines without any basal plate of bone. The main organ of propulsion seems to have been the heterocercal tail, while the broad anterior region passes out on each side into a flap-like portion which may represent a pectoral fin. On the under surface of Thelodus there occur trans-verse markings which probably are caused by the presence of a branchial apparatus of the ordinary Selachian type. In the Drepanaspidae (Lower Devonian) and Pteraspidae (Upper Silurian and Lower Devonian) the isolated placoid elements of the Coelolepidae have undergone fusion to a less or greater extent into large plates, which ensheath the anterior body region, the posterior portion possessing rhombic scales. The Ostracoderms so far mentioned are grouped together under the name Heterostraci. The Osteostraci form another main division of the Ostracoderms, distinguished from the Heterostraci by the presence of true unmodified bone in their skeletal plates. The orbits are more dorsal in position and a dorsal fin is known to occur, while none has as yet been recognized in the Heterostraci. The most familiar members of the group are the Cephalaspidae of the Silurian and Devonian with their highly characteristic crescentic shield covering the dorsal side of the head region. From behind the posterior horns of this shield there project in some specimens paddle-like structures which may be pectoral fins, or possibly structures serially homologous with limbs and not represented in modern Selachians. Among the less doubtful members of the Selachii among fossil forms first place must be given to the Pleuropterygii represented by the genus Cladoselache (8) from the Upper Devonian of Ohio. This was a shark-like creature with the mouth apparently terminal. The body was covered with shagreen placoid elements: . there were a series (five or seven) of gill slits on each side and the, skull was probably hyostylic. The notochord was apparently persistent. The chief interest of Cladoselache, however, lies in its paired fins which are held by upholders of the " lateral fold " theory to be remarkably primitive. The unpaired fins are obviously highly developed—the tail being almost homocercal with a lateral keel on each side as in various- existing sharks, and it seems on the whole unlikely that the paired fins should be very primitive while the unpaired fins are so highly developed. Moreover, the facts of structure of the paired fins so far as at present known seem to fit in quite well with the view that they are modifications of the uniserial archipterygial type (see ICHTHYOLOGY, fig. 2). From Challenger Reports Zool.. published by H.M. Stationery Office. (After Gunther.) The Ichthyotomi, including the family Pleuracanthidae (Lower Carboniferous to Permian), are again of special interest as regards their paired fins which are obviously of the uniserial archipterygial type. The tail is protocercal and the mouth nearly terminal. The Acanthodei are small fishes ranging from the Upper Silurian to Permian. They had strongly heterocercal tail, gill clefts apparently opening independently to the exterior, but they are specially characterized by the strong spines in front of each fin and by the calcified plates lying superficial to the cranium, jaw apparatus and pectoral girdles. (J. G, K.)
End of Article: SELACHIANS, or ELASMOBRANCHII
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