x FINS AND SKIN 433 



already mentioned, it is represented in the tadpole 

 but disappears at metamorphosis. 



Springing from the body are a number of flattened 

 folds, called the fins, divisible into median and paired. 

 The median folds are not continuous, as in the tadpole 

 and lancelet, but are subdivided into several distinct 

 parts, viz., into two dorsal /ins along the middle line of 

 the back, a caudal fin lying along the ventral edge of the 

 upturned or heterocercal tail, and a ventral fin behind 

 the anus. The paired folds are the pectoral fins, situated 

 one on either side of the trunk just behind the last gill- 

 cleft, and the pelvic fins, one on each side of the vent : 

 these correspond to the pectoral and pelvic limbs of 

 the frog. In the male there is connected with the inner 

 border of each pelvic fin a grooved, rod-like structure 

 known as the clasper, which serves as a copulatory organ. 



It is very possible that the paired fins, like the median 

 fins, are specialised portions of a primarily continuous 

 fin-fold which extended along either side of the body, like 

 the lateral or metapleural fold of Amphioxus (p. 421). 



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 part of the body-wall consists, 

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

 or epiderm (Fig. in, Der. Epthm), formed, like that of 

 the frog, of several layers of cells ; and an inner layer 

 of connective-tissue, the derm. In the derm are in- 

 numerable close-set, calcified bodies, each consisting of 

 a little irregular bony plate produced into a short spine 

 composed, like the teeth present in the frog and most 

 Vertebrates, of a calcified tissue harder than bone known 



