258 "XJrfAR'ACTfcitS'bF VERTEBRATE ANIMALS 



to the tail-fin, which is unsymmetrical, consisting of a large upper 

 lobe into which the slender end of the body is continued, and a 

 smaller lower lobe. Such unequal or heterocercal (Gk. heteros, 

 diverse; kerkos, tail) tails are shown by reference to fossil forms 

 to be of very ancient type. There is good reason to believe that 

 remote fish-ancestors had a continuous unpaired fin, of which the 

 existing ones are fragments which have been retained and 

 enlarged to suit special purposes. The Dog- Fish, however, also 

 possesses paired fins, consisting of two large pectorals in front 

 and two smaller pelvics placed close together farther back. These 

 are the equivalents of the fore- and hind-limbs of the terrestrial 

 vertebrates so far described, but differ in important respects in 

 accordance with differences in use. The limbs of a Salamander, 

 for instance, have to support the weight of the body and are the 

 means of progression. The presence of digits is of obvious 

 advantage as regards the former, while locomotion would be 

 difficult and awkward were the limbs not transversely divided 

 into regions capable of being moved upon one another. But the 

 limbs of an ordinary fish do not support the body, and their chief 

 use seems to be that of steadying it in the water and directing 

 its movements. For these purposes the undivided paddle-like 

 shape which we associate with the paired fins of a fish appears 

 best adapted. 



The large curved mouth is situated on the under side of the 

 head, and not far in front of it are the rounded nostrils, each of 

 which is connected by a groove with the corresponding corner 

 of the mouth. Far back on the under surface of the body, and 

 marking the junction of trunk and tail, is a rounded cloacal 

 aperture situated between the pelvic fins. On each side of this 

 opening is a small aperture known as an abdominal pore, of 

 unknown use, but commonly found in more than one group of 

 fishes. The oblique cat-like eyes are provided with imperfectly 

 movable upper and lower eyelids. A tadpole, it will be remem- 

 bered, has four gill-slits on each side of the throat, the cavity 

 of which is thus placed in communication with the exterior. Here 

 there are five gill-slits, and also a superseded gill-slit known as 

 the spiracular cleft and opening behind the eye by a small round 

 hole, the spiracle. 



The skin is of a brownish hue, much darker above than 

 below, and marked with good-sized roundish spots. Projecting 



