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. Epthni) 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 

 acoelomate in the adult. The coelomic epithelium (Cal. 

 Epthm, Ccel. Mpthm') 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 



