514 PISCES— SUB-CLASS IV.—ELASMOBRANCHII. 



Although the structure of the breathing apparatus alone is sufficient to dis- 

 tinguish the Elasmobranchii from all other fishes, it is advisable to glance at 

 some of the other leading structural features of their organisation. And 

 here it may be mentioned that we allude only to the living forms, and that 

 some extinct groups do not agree in all respects with the characteristics of 

 the sub-class as defined from the former alone. From the typical bony 

 fishes, the Elasmobranchii are distinguished internally by the cartilaginous 

 nature of the skeleton ; such hardening as takes place in this framework 

 being due to the deposition of calcareous matter in the cartilage, and not to 

 the replacement of the latter by bone. In consequence of this want of 

 development of bony structures, the primitive cartilaginous skuU persists 

 throughout life ; and the functional jaws are bars of cartilage, which do not 

 correspond with the bony jaws of higher creatures. Teeth of the ordinary 

 structure are developed on these spurious jaws; and such hard structures as 

 exist in the skin likewise partake of the nature of teeth, and are, therefore, 

 very distinct from bone. Of this kind are the fine granules found in the 

 skin of sharks, and the larger pustules, cones, knobs, or spines occurring on 

 that of many kinds of rays, where they are often arranged in a number of 

 isolated prominent ridges on different parts of the back. From this remark- 

 able absence of bone in the group, the name of boneless fishes would not be 

 an inappropriate name for the sharks and rays. Owing to the number of 

 minute tooth-like structures embedded in its surface, shark-skin forms an 

 admirable polisher; and as it also afibrds a firm hand-hold, it is likewise used 

 for covering sword-hilts. 



In their internal structure the Elasmobranchii show a decidedly primitive 

 type, in that where the two nerves supplying the eyes meet, there the fibres 

 interlace to form a chiasma. The formation of a spiral valve by the lining 

 membrane of the intestine also seems to be a primitive feature. In no mem- 

 ber of the group is an air-bladder ever developed. Unlike ordinary tishes, 

 sharks lay a small number of eggs, each of which is of large size, and is 

 entirely separate from the other. Generally these eggs are invested in a 

 hard, horny envelope, which is of an oblong shape, and furnished at each 

 angle with a curling tendril, by means of which they become moored to the 

 stems of sea-weeds or other submarine objects, where they remain till the 

 young come forth. The empty envelopes are frequently to be found thrown 

 up on the beach, and are commonly known as sea-purses. In certain species 

 the eggs are, however, retained within the body of the female parent untQ 

 hatched, so that the young are born alive. While within the shell, young 

 sharks and rays are provided with external gills, and thus resemble tadpoles, 

 but these gills are always lost before birth. The upper surface of the head 

 in many members of the group is furnished with the organs known as 

 spiracles. 



As regards the internal skeleton of the fins of the Elasmobranchii, it will 

 suffice for our present purpose to state that the supporting cartilages are 

 arranged somewhat in the form of a fan made up of flattened du'ergent rays, at 

 the base of which are certain other short cartilages. To the homologies of 

 these latter it is unnecessary to refer in this work. In a few living sharks 

 powerful spines— whose structure is also comparable to that of teeth— form 

 the front portion of the dorsal tins, such spines being loosely inserted in the 

 flesh, without any basal connection with the vertebnB. The pelvic fins are 

 situated a long distance behind the pectoral pair; and in the males the sup- 

 portuig axis of each of the former is developed, as in the Chimreroids, into a 



