XXIX EXOSKELETON 369 



line. The whole apparatus, together with other canals in the 

 head, is really a system of tubes sunk in the skin, and con- 

 stitutes an important, but imperfectly understood, sensory 

 organ. 



Springing from the body are a number of flattened folds, 

 called the fins, divisible into median and paired. The 

 median folds are two dorsal fins {d.f. i, d.f. 2) along the 

 middle line of the back, a caudal fin {cd.f) lying mostly 

 along the ventral edge of the upturned tail, and a ventral 

 fin {v. f) behind the anus. The paired folds are the pectoral 

 fins {pcf.f), situated one on each side of the trunk just 

 behind the last gill-cleft, and the pelvic fins, one on each 

 side of the anus. The pectoral and pelvic fins are the 

 paired appendages or limbs of the dogfish : as in other 

 Vertebrates there are only two pairs, the pectorals corre- 

 sponding with the fore-limbs, the pelvics with the hind- 

 limbs of the higher forms. 



The fish swims by vigorous strokes of the tail : the 

 pectoral fins are used chiefly for steering, and the dorsal 

 and ventral fins serve, like the keel of a boat, to maintain 

 equilibrium. 



The skin or external layer of the body-wall consists, as 

 usual, of two layers, an outer layer of deric epithelium 

 (Fig. 99, Der. Epthni) differing from that of previous types 

 in being formed of several layers of cells, and an inner 

 layer of connective tissue, the dermis. In the dermis are 

 innumerable close-set calcareous bodies (Fig. 99, Derm. Sp), 

 each consisting of a little irregular plate of bone produced 

 into a short enamelled spine, which projects through the 

 epidermis and gives a rough, sand-paper-like character to 

 the skin. These placoid scales or dermal teeth together 

 constitute the exoskeleton of the dogfish : it is a discon- 

 tinuous dermal exoskeleton like that of the starfish. 



Beneath the dermis is the muscular layer m which we 



R B 



