FINS AND SKIN 433 



tegumentary sense-organ : 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 fins along the middle line of the back, 

 a caudal fin lying along the ventral edge of the up- 

 turned 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 clasfier^ ^ 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. 



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 con- 

 nective-tissue, the derm. In the derm are innumerable 

 close-set, calcified bodies, each consisting of a little irregular 

 plate of bone produced into a short spine composed, like 

 the teeth present in the frog and most Vertebrates, of a calci- 



PRACT. ZOOL. F F 



