C Batides. This group includes the Rays and Skates, and is distinguished by the fact that the branchial apertures are placed on the under surface of the body, forming two rows of openings a little behind the mouth. In the typical members of the group the body is flattened out so as to form a kind of rhomboidal disc (fig. 271), the greater part of which is made up of the enormously-developed pectoral fins. The pectoral fins are united by cartilage with the skull, and there is no median facet upon the occiput for articulation with the first vertebra. Upon the upper surface of the disc are the eyes and spiracles; upon the lower surface are the nostrils, mouth, and branchial apertures. The flattened bodies of the Rays, however, must be carefully distinguished from those of the Flat-fishes (Pleuronectidae). In the former the flat surfaces of the body are truly the dorsal and ventral surfaces. In the latter, as before remarked, the body is flattened, not from above downwards, but from side to side, and the head is so twisted that both eyes are brought to one side of the body. The tail in the Rays is long and slender, usually armed with spines, and generally with two or three fins (the homologues of the dorsal fins). The mouth is often paved with flat teeth, of a more or less rhomboidal shape.

The typical members of the Batides are the Skates and Rays, of which the common Thornback (Rata clavata) may be taken as a familiar example. More remarkable than the common Rays is the Electric Ray or Torpedo, which has the power of discharging electrical shocks if irritated. The identity of the force produced in this way with the electricity of the machine has been demonstrated by many careful experiments. The Torpedo owes its remarkable power to two special organs - the "electrical organs," which consist of two masses placed on each side of the head, and consisting each of numerous vertical gelatinous columns, separated by membranous septa, and richly furnished with nerves from the eighth pair ; the whole arrangement presenting a singular resemblance to the cells of a voltaic battery. There is no doubt, however, but that the force which is expended in the production of the electricity is only nerve-force. For every equivalent of electricity which is generated, the fish loses an equivalent of nervous energy ; and for this reason the production of the electric force is strictly limited by the amount of nerve-force possessed by the animal. • Other well-known members of the family are the Sting-rays (Trygon), the Eagle-rays (Myliobatis), the Horned Rays (Cephaloptera), and the Beaked Rays (Rhinobatis).

Fig. 271.   Batides. Rata margtnata, one of the Skates. Reduced one   sixth. (After Gosse.)

Fig. 271. - Batides. Rata margtnata, one of the Skates. Reduced one - sixth. (After Gosse.)

In the Saw-fish (Pristis antiquorum) the body has not the typical flattened form of the Rays, and the snout is elongated so as to form a long, sword-like organ, the sides of which are furnished with strong tooth-like spines. This constitutes a powerful weapon, with which the Saw-fish attacks the largest marine animals. Though principally inhabiting the sea, the Saw-fishes are not wholly marine, fresh-water forms occurring in Nicaragua and in the Philippines.

Before leaving the Elasmobranchii, a few words may be said as to their position in the class of fishes. From the cartilaginous nature of the en-doskeleton, and the similarity between the form of their gills and those of the Lampreys and Myxinoids, the Elasmobranchii were long placed low down in the scale of fishes, to which also the permanently heterocercal tail conduced. When we come, however, to take into consideration the sum of all their characters, there can be little hesitation in placing the order nearly at the summit of the entire class. The nervous system, and especially the cerebral mass, is very much more highly developed proportionately than is the case with any other division of the fishes. The organs of sense are, comparatively speaking, of a very high grade of organisation, the auditory organs being more than ordinarily elaborate, the eyes being sometimes furnished with a third eyelid (membrana nictitans), and the nasal sacs having a very complex structure. The structure of the heart agrees with that of the Ganoids, and is a decided advance upon the heart of the more typical bony fishes. Finally, the embryo, before its exclusion from the egg, is furnished with external filamentous branchiae, this being a decided approximation to the Amphibia.