3 



13 



SKIN 



Upon the back and sides of the rough hound the skin is of a 

 grey-brown colour with small spots of darker brown ; upon the 

 belly it is whitish. It changes colour slowly to match its back- 

 ground, control being by the pituitary gland (p. 367). The outer 

 part of the skin is largely composed of the protein keratin, and 

 although it feels smooth to the hand if it be stroked from head to 

 tail, it is rough if it be stroked in the opposite direction. This is due 

 to the presence of scales, which are not flat like those of most 

 fishes, but bear minute spines directed backwards (Fig. 236). Each 

 consists of a calcified basal plate embedded in the dermis, and a 

 spine which is composed of dentine covered with enamel. A pulp 



B 



,-e.i 



A 



Fig. 236. — Placoid scales. 



A, A portion of the skin of the rough hound as seen under a hand lens ; B, a single scale removed from tl.c 



skin ; C, the same in section (diagrammatic). 

 b., Base of the scale ; c, the same in section ; d., dentine ; e., enamel ; p., pulp cavity. 



cavity, containing highly vascular connective tissue, passes 

 through the base into the spine. The structure is thus that of a 

 tooth (p. 362) and scales of this type are called placoid. Teeth 

 must be looked on from the evolutionary point of view as modified 

 placoid scales. The teeth of the dogfish, which differ from the 

 scales only in size, form an intermediate link. 



MUSCLES AND MOVEMENTS 



In the body-wall (Fig. 442) the muscles are for the most part 

 segmentally arranged, each muscle-segment being known as a 



M.Z. — II 



