THE DOGFISH 



343 



do not open into an atrium ; (4) they have a heart, which 

 lies below the gut ; (5) most, though not all of them, possess 

 two pairs of limbs and none has more. 



Upon the back and sides of the rough hound the skin 

 Ski|) is of a grey-brown colour with small spots of 



darker brown; upon the belly it is whitish. It 

 feels smooth to the hand if it be stroked from head to tail, but 

 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. 



mym. 



Fig. 247. — Part of the tailof-a-dogfish seen from the left side, with a 

 piece of the skin removed. 



I.I., Tube of the lateral line ; myc. t myocommata or septa of connective tissue ; 

 mym., myomeres. 



Such scales are said to be placoid. Each consists of a 

 calcified basal plate embedded in the dermis, and a spine 

 which is composed of dentine covered with enamel. A 

 pulp cavity, containing highly vascular connective tissue, 

 passes through the base into the spine. . It will be seen 

 that the general features of such a scale resemble those of 

 the tooth of a frog. In fact the teeth of the dogfish, 

 though they are larger, have the structures of the scales, 

 and we must regard teeth as modified scales. 



The muscles of the body-wall are for the most part 

 segmentally arranged, each muscle-segment being known 

 as a myomere. The myomeres do not lie straight, but 



