Elasmobranch

“ The brain and the reproductive organs agree more closely with the corresponding structures in the Elasmobranchs than with those of any other Fishes, and the agreement extends to the large size of the eggs and their enclosure in horny egg-cases. In both groups the nostrils are connected with the mouth by oro-nasal grooves; the hyoidean hemibranch is a true gill, and there is no air-bladder. The Holocephali also agree with the Elasmobranchs in retaining such primitive features as an intestinal spiral valve and a conus arteriosus. ”

“ The Greenland Shark (Laemargus borealis) is unique amongst Elasmobranchs. Its eggs are small and unprotected by egg-cases, and their fertilisation is said to be effected in the water after deposition, as in the generality of Fishes.
Fossil remains of Elasmobranchs in the shape of fin-spines (ichthyodorulites) and dermal denticles, associated with various Ostracodermi (Coelolepidae, Pteraspidae, and Cephalaspidae) , are amongst the earliest undoubted indications of Vertebrate life. ”

“ The area of respiratory surface tends to become increased by the development of outgrowths. Frequently these take the form of regular plate-like structures known as gill lamellae. In the Selachians these lamellae are strap-like structures (Elasmobranch) attached along nearly their whole length to the gill septum as shown in fig. ”

“ As the peculiar breathing apparatus of the fish is adapted to its element and mode of life, so is every part and appendage of the whole structure. Adaptation reigns through the whole. The elongated tapering body, its scaly covering exquisitely adjusted, the material and position of the fins, all attest the intelligent and beneficent purpose of the Creator.
The Elasmobranchii [36] (strap-gilled) have a cartilaginous skeleton, rough skin, and uncovered gills. The Shark, the Saw-fish and the Ray are representatives of this order. ”

“ The Elasmobranchs are for the most part active predaceous Fishes, living at different depths in the sea, from the surface to nearly a thousand fathoms, and ranging from mid-ocean to the shallower waters round the coasts in almost every part of the world. Although typically marine, they sometimes ascend rivers beyond the reach of tides, and a few are permanent inhabitants of fresh water. They are most abundant in tropical and subtropical areas, where they also attain their greatest size, and are numerous in temperate regions, but there are some species which are typically Arctic. ”

“ In Elasmobranchii a similar fold to that in Ammocœtes first makes its appearance in the embryo. This fold is from the first not quite straight, but winds in a long spiral round the intestine. In the course of development it becomes converted into a strong ridge projecting into the lumen of the intestine (fig. 388, l) . The spiral it makes becomes much closer, and it thus acquires the form of the adult spiral valve. A spiral valve is also found in Chimæra and Ganoids. No rudiment of such an organ is found in the Teleostei, the Amphibia, or the higher Vertebrata. ”

“ If it be taken into consideration that the continuous lateral fin, of which the rudiment appears in Elasmobranchii, does not exist in any adult Vertebrate, and also that a continuous dorsal fin exists in many Fishes, the small differences in development between the paired fins and the dorsal fins will be seen to be exactly those which might have been anticipated beforehand. Whereas the continuous dorsal fin, which often persists in adult fishes, attains a considerable development before vanishing, the originally continuous lateral one has only a very ephemeral existence. ”

“ Through shark-like forms all the higher vertebrates must probably trace their descent. Sharks' teeth and fin-spines are found in all rocks from the Upper Silurian deposits to the present time, and while the majority of the genera are now extinct, the class has had a vigorous representation in all the seas, later Palæozoic, Mesozoic, and Cenozoic, as well as in recent times.
Most of the Elasmobranchs are large, coarse-fleshed, active animals feeding on fishes, hunting down their prey through superior strength and activity. ”

“ In the seas Elasmobranchs and Teleosteans exist in swarming numbers side by side, but it is impossible to say that one type is more adapted to marine life than the other. There is good reason to believe that bony fishes were evolved from Elasmobranchs in fresh water which was shallow and foul, so that lungs were evolved for breathing air, and that marine bony fishes are descended from fishes with lungs; but no reason has been given for the evolution of bone in place of cartilage or for the various kinds of scales. ”

“ Fig. 74.—Teeth of Shark.
In the Ganoids and bony fishes there is a gill cover, and in all but a few Ganoids there is some formation of true bone, whereas [98] in the Elasmobranchs the skeleton is wholly cartilaginous. One of the most striking anatomical features of these groups, and one which distinguishes them from the Elasmobranchs, is the presence of a swim bladder, a large sac-like outgrowth from the upper part of the gut. The function of the swim bladder is that of regulating the specific gravity of the fish, which becomes greater or less according as air is expelled or taken in. ”

“ Their gill-clefts are covered by an operculum, and they have a well-developed air-bladder. Whilst they are in many respects more highly developed than the Elasmobranchs, and are intimately connected with the typical Ganoids and other bony fishes (all of which form a great, manifold side-branch of the general vertebrate stem) , they stand in many other respects (notably, the structure of the paired fins, the vertebral column, and the air-bladder) nearer the main-stem of our own ancestral line. ”

“ In the Elasmobranchii the primitive ova exhibit in a hardened state an irregular form which might appear to indicate that they possess a power of altering their shape, a view which is further supported by some of them being at the present stage situated in a position very different from that which they eventually occupy, and which they can only reach by migration. ”

“ In the anterior or visceral part of the body the haemal arch is split and its two sides spread out deep to the muscles and lying between them and the coelom to form the ribs. In the elasmobranchs on the other hand the ribs lie among the muscles as they do in higher vertebrates, and the fact that both kinds of ribs are coexistent in the same segments in the interesting and archaic Nilotic fish Polypterus bichir shows that they are de- veloped independently of one another. The sternum is never found in fishes with the possible exception of the comb-toothed shark (Notidanus) . ”

“ The remainder of the primitive streak represents, in a rudimentary form, the linear streak in Elasmobranchii, formed by the coalesced edges of the blastoderm, which connects the hinder end of the embryo with the still open yolk blastopore. That it is in later stages not continued to the edge of the blastoderm, as in Elasmobranchii, is due to its being a rudimentary organ. The more or less complete fusion of the layers in the primitive streak is simply to be explained by this structure representing the coalesced edges of the blastopore ”

“ Even when the liability to waste is very great, compensation may be afforded by exceptional fecundity. The copulation of the sexes and the internal fertilisation of the eggs occur only in Elasmobranchs and some Teleosts. The copulatory organs of Elasmobranchs are the so-called "claspers" with which the males are provided. ”

“ A few vestigial branchial lamellae may be developed on the anterior wall of each spiracle in Acipenser and Polyodon, but are wanting in Polypterus, and, as in Elasmobranchs, represent a mandibular or spiracular pseudobranch.
The structure usually regarded as a hyoidean hemibranch in the Teleostomi differs greatly in its development in different members of the group. In Acipenser it is undoubtedly the hemibranch of the hyoid arch and is a true gill, receiving venous blood from the ventral aorta and returning arterial blood to the dorsal aorta, as in Elasmobranchs. ”

“ Elasmobranchs.—Among sharks and skates the usual number of segments is from 100 to 150 and upwards. In the extinct species as far as known the numbers are not materially different. The Carboniferous genus, Pleuracanthus, has about 115 vertebræ. The Chimæras have similar numbers; Chimæra monstrosa has about 100 in the body and more than as many more in the filamentous tail. ”

“ In Elasmobranchs the meso-pterygium is missing, the propterygium is small or absent, and the fin is mainly composed of the meta-pterygium [132] (generally called basi-pterygium) and its radiale. The males in Elasmobranchii and Holocephali have the distal end of the meta-pterygium prolonged into a clasper.
In Ganoids and in Teleosteans the loss of the pelvic girdle causes the pelvic fin to be still further removed from the primitive state. There is always a large basi-pterygium which lies imbedded in the muscles and meets its fellow at its proximal end. ”

“ The Amphibia are much more closely related to certain fishes than the classes of fishes are to each other. Still for purposes of systematic study, the frogs and salamanders are left out of the domain of ichthyology, while the Tunicata and the Enteropneusta might well be included in it.
The known branchiferous or gill-bearing chordates living and extinct may be first divided into eight classes—the Enteropneusta, the Tunicata, the Leptocardii, or lancelets, the Cyclostomi, [Pg 462] or lampreys, the Elasmobranchii, or sharks, the Ostracophori the Arthrodira, and the Teleostomi, or true fishes. ”

“ With their formation the trunk rapidly increases in length. At their outer border the segmental duct (fig. 50 B, and fig. 52 A, Sd) is very early established. It is formed, as in Elasmobranchs, as a solid outgrowth of the mesoblast (fig. 51, Wg) ; but its anterior extremity becomes converted into a pronephros (fig. 57, pr.n) .
[Pg 107] Before hatching, the embryo has to a small extent become folded off from the yolk both anteriorly and posteriorly; and has also become to some extent vertically compressed. ”

“ It might have been anticipated that the Teleostean Ganoids would resemble the Teleostei, but, from an examination of adult Teleostei, it would seem to be clear that the relations of the parts are the same as in Elasmobranchii, i.e. that the ribs have no share in forming the hæmal canal in the tail. Aug. Müller and Götte have however brought embryological evidence (though not of a conclusive character) , to shew that in the embryo the ribs really fuse with the hæmal processes in the tail, and so assist, as in the Ganoids, in forming the hæmal canal. ”

“ These external gills, which differ fundamentally from the external gills of Elasmobranchii in being covered by epiblast, soon elongate and form branched ciliated processes floating freely in the medium around the embryo (fig. 80) .
Before hatching the excretory system begins to develop. The segmental duct is formed as a fold of the somatic wall at the dorsal side of the body cavity (fig. 79, u) . Its anterior end alone remains open to the body cavity, and gives rise to a pronephros with two or three peritoneal openings, opposite to which a glomerulus is formed. ”

“ Hasse's Classification of Elasmobranchs.—In 1882, Professor Carl Hasse proposed to subdivide the sharks on the basis of the structure of the individual vertebræ. In the lowest group, a hypothetical order of Polyospondyli, possibly represented by the fossil spines called Onchus, an undivided notochord, perhaps swollen at regular intervals, is assumed to have represented the vertebral column. ”

“ It has already been shewn that in all the higher Vertebrata muscle-plates appear, which closely resemble those in Elasmobranchii; so that all the higher Vertebrata pass through, with reference to their muscular system, a fish-like stage. The middle portion of the inner layers of their muscle-plates becomes, as in Elasmobranchii, converted into muscles at a very early period, and the outer layer for a long time remains formed of indifferent cells. ”

“ The presence of both kinds of fin-rays, or of soft rays only, is one of the more obvious distinctions between the Teleostean groups of the Acanthopterygii and the Malacopterygii, of which the Perch and the Salmon respectively are well-known examples. Powerful spines are frequently developed in front of the dorsal fin in many living and extinct Elasmobranchs, and, under the general term of "ichthyodorulites," constitute the sole fossil remains of many extinct Devonian and Carboniferous genera. ”

“ They are of small size, and the number of individuals preserved is very large. The characteristic genera are—Catopterus, Ischypterus, Ptycholepis, Rhabdolepis, and Turseodus. From the Jurassic deposit no remains of fishes are known, [4] but in the Cretaceous, ichthyic life assumed many and various forms; and the first representatives of the Teleosts, or bony fishes, the characteristic fishes of to-day, make their appearance. In the deep open sea of this age, Elasmobranchs were the prevailing forms, Sharks and Chimæroids being most numerous. ”

“ So far, however, as their mode of propagation is known, it may confidently be affirmed that the development of the majority of the species of Eleutherozoa is of the second or larval type. In this type there is little food-yolk in the egg, and the young animal or larva is forced from a very early period of development to seek its own living, and hence it is usually a considerable time (from a fortnight to two months) before the adult form is attained. ”