APPENDAGES 321 



Appendages are present, but in a very different form from 

 those .of the crayfish. They consist of flat processes of the 

 body- wall called fins. Two of them (D.F 1 , D.F") are 

 situated in the middle line of the back (dorsal fins] : one 

 ( V.F} in the middle ventral line behind the anus (ventral 

 fin\ and one (C.J?) is attached to the up-turned end of the 

 tail (caudal fiii) : all these being unpaired structures or 

 median fins. Then there is a pair of pectoral fins situated 

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

 pelvic fins placed one on each 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. Rpthm) composed of several layers of 

 cells, and of an inner connective tissue layer or dermis 

 (Demi}. 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 these externally. 



The large coelome (cceV) is confined to the trunk : it is 

 characteristic of vertebrates that both head and tail are 

 accelomate in the adult. The coelomic epithelium (Ccel. 

 Epthm, Ccvl. Epthm 1 ) 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. 



Y 



