This order includes the Sharks, Rays, and Chimaerae, and corresponds with the greater and most typical portion of the Chondropterygidae or Cartilaginous fishes of Cuvier. The order is distinguished by the following characters: The skull and lower jaw are well developed, but there are no cranial bones, and the skull consists of a single cartilaginous box, 7vithout any indication of sutures. The vertebral column is sometimes composed of distinct vertebrae, sometimes cartilaginous or sub-notochordal. The exoskeleton is in the form of placoid granules, tubercles, or spines. There are two pairs of fins, representing the limbs, and supported by cartilaginous fin-rays; and the ventral fins are placed far back near the anus.
The pectoral arch has no clavicle. The heart consists of a single auricle and ventricle, and the bulbus arteriosus is rhythmically contractile, is provided with a special coat of striated muscular fibres, and is furnished with several transverse rows of valves.
The gills are pouch-like.
In most of the above characters, it will be seen at once that the Elasmobranchii agree with the Ganoid fishes, especially, as regards the structure of the heart. The following points of difference, however, require more special notice:
I. The exoskeleton is what is called by Agassiz "placoid." It consists namely, of no continuous covering of scales or ganoid plates, but of more or less numerous detached grains, tubercles, or spines, composed of bony or dentinal matter, and scattered here and there in the integument. In the case of the Rays, these placoid ossifications often take a very singular shape, consisting (fig. 247, c) of an osseous or cartilaginous disc, from the upper surface of which springs a sharp recurved spine, composed of dentine. The so-called "shagreen" of the Dog-fishes and Sharks is composed of very small and close-set tooth-like processes (fig. 247, d). At other times the placoid structures are developed into "dermal defences" or " ichthyo-dorulites." The minute structure of these exoskeletal structures is closely or entirely similar to that of the teeth. In some cases the exoskeleton is absent.
II. The gills are fixed and pouch-like, and differ very materially from those of the Bony and Ganoid Fishes. In the case of the Sharks and Rays, the structure of the gills is as follows : The branchial arches are fixed, and the branchial laminae are not only attached by their bases to the branchial arches, but are also fixed by the whole of one margin to a series of partitions, which divide the branchial chamber into a number of distinct pouches (fig. 268). Each partition, therefore, carries a series of branchial laminae attached to each side, like the leaves of a book. By means of these septa a series of branchial sacs or pouches are formed, each of which opens internally into the pharynx by a separate slit, and communicates externally with the water by a separate aperture placed on the side of the neck (fig. 268, B). The arrangement of the gills being such, there is, of course, no gill-cover, and no bran-chiostegal membrane or rays. In one section of the order, however - viz., the Holocephali - though the internal structure of the gills is the same as the above, there is only a single branchial aperture or gill-slit externally, and this is protected by a rudimentary operculum and branchiostegal rays.
III. Another character in the Elasmobranchii, shared, however, by many of the Ganoids, is the structure of the intestinal canal. The intestine is extremely short; but, to compensate for this, there is a peculiar folding of the mucous membrane, constituting what is known as the "spiral valve." The mucous membrane, namely, from the pylorus to the anal aperture, is folded into a spiral reduplication, which winds in close coils round the intestine, like the turns of a screw. By this means the absorptive surface of the intestine is enormously increased, and its shortness is thus compensated for.
Fig. 268. - A, Head of Piked Dog-fish (Spinax), showing the transverse mouth on the under surface of the head, and the apertures of the gill-pouches. B, Diagram of the structure of the gill-pouches: o o External apertures; i i Apertures leading into the pharynx; s s Gill-sacs, containing the fixed gills.
It is to be noted that some high authorities are in favour of the view that the Elasmobranchii are to be regarded as a distinct class, and not as merely an order of the class Pisces.
The order Elasmobranchii is divided into two sub-orders : the Holocephali, characterised by the mouth being terminal in position, and there being only a single gill-slit; and the Plagio-stomi, in which the mouth is transverse, and placed on the under surface of the head (fig. 268, A), and there are several branchial apertures on each side of the neck.