544 



CLASSIFICATION OF VERTEBRATES 



Superclass II— Gnathostomata 

 The remaining vertebrates agree in possessing jaws, and are 

 further distinguished from the Cyclostomata in having a double 

 instead of a single nasal opening, and in possessing three, instead 

 of two, semicircular canals. They are now generally placed in 

 seven or eight classes, the former class Pisces, the fish, being 

 divided into three or four groups which represent early diver- 

 gences of the gnathostome stock. 



class I — CHONDRICHTHYES OR ELASMOBRANCHII 



These are the cartilaginous fishes, and include the modern sharks, 

 of which the dogfish is a good example, and rays and skates, in 

 which the body is flattened and the tail reduced. They are difficult 

 to define, but some of the chief characters of the modern forms are 

 the absence of bone from the endoskeleton ; a valvular contractile 

 conus arteriosus ; the presence of a large amount of urea in the 

 blood ; plate-Hke gills ; absence of an operculum ; a spiracle ; 

 absence of an air bladder ; a heterocercal tail (p. 552) ; a spiral 

 valve in the intestine ; claspers in the male ; and large yolky eggs 

 laid in heavy cases. The extinct forms did not possess all these 

 features. The only near-diagnostic character is the placoid scale, 

 which has essentially the same structure as a tooth and contains 

 dentine. If this material is regarded as a type of bone (p. 518) it is 

 untrue to say that the elasmobranchs contain no bone. A number 

 of Silurian and Devonian fossils have a bony exoskeleton, under- 

 neath which have been traced blood vessels and nerves of an 

 elasmobranch pattern ; whether these fishes are regarded, as at 

 first they were, as primitive Chondrichth3'es, or whether they are 

 placed in a separate class, the Placodermi, does not much matter, 

 but it is clear that the earliest known gnathostomes had much 

 bone and that the condition of the present-day cartilaginous 

 fishes is probably secondary. Their loss of armour may be con- 

 nected with the development of actively predaceous habits. In 

 spite of their specialisation in this direction the modern sharks 

 represent, in most of their soft parts, the best that we can find 

 as an example of a primitive vertebrate. It has been suggested 

 that cartilage developed as an embryonic tissue ; if this is so either 

 the skeleton of the dogfish represents the original vertebrate 

 embryonic skeleton, or its resemblances to the embryonic skeleton 

 of higher vertebrates are due to convergence. 



