xxvn GENERAL CHARACTERS 325 



one on each side just behind the last gill-cleft, and a pair of 

 pelvic fins placed one on either side of the vent : these are 

 the lateral or paired fins. It is characteristic of Vertebrata 

 that the number of lateral appendages never exceeds two 

 pairs. 



The skin or external layer of the body-wall consists of an 

 outer epidermis (Der. Epthm) composed of several layers of 

 cells, and of an inner connective tissue layer or dermis 

 (Derm}. In the latter are found innumerable bony scales 

 (Derm. Sp) constituting a dermal exoskeleton. The muscular 

 layer of the body-wall ( M) is of great thickness, especially 

 in the dorsal region, and is distinctly segmented, indicating 

 that the body of the dog-fish, like that of Polygordius and 

 the crayfish, is divisible into metameres, although there is no 

 indication of them externally. 



The large ccelome (Cat) is confined to the trunk : it is 

 characteristic of vertebrates that both head and tail are 

 accelomate in the adult. The ccelomic epithelium (CceL 

 Epthm, Cal. Epthui} is underlaid by a distinct layer of 

 connective tissue, the two together forming the peritoneum. 



Another important vertebrate character is that the dorsal 

 region of the body-wall contains a median longitudinal 

 canal (C. Sp. Cav.} extending from shortly behind the snout 

 to near the end of the tail. This is the cerebro-spinal cavity 

 and contains the central nervous system. 



Still another characteristic feature is the presence, in 

 addition to the dermal exoskeleton, of an endoskeleton^ or 

 system of internal supporting structures. Between the 

 cerebro-spinal cavity above and the ccelome below is a 

 longitudinal series of biconcave discs or vertebral centra 

 ( V. Cent] : they are formed of a peculiar tissue called 

 cartilage or gristle, and are strongly impregnated with lime- 

 salts : in the young condition their place is occupied by a 



