8 THE SKATE 



will divide the animal into two mirrored halves. The 

 structures of a vertebrate therefore are either paired, in 

 which case they are located symmetrically on each side, or 

 are unpaired, in which case they are located in* the median 

 plane. 



II. EXTERNAL STRUCTURES. 



1. Parts of the Body. 



The body is divided into a greatly flattened anterior 

 portion consisting of the head and the trunk, and a slender 

 posterior portion, the tail. This broad, flat form is char- 

 acteristic of bottom-feeding fishes and is obtained by a 

 shortening of the dorsoventral axis. For a generalized 

 body form see the shark which is on demonstration. 



2. Fins. 



The skate has median or unpaired fins and paired fins. 

 Locate the two small dorsal fins on the tail; a pair of 

 pectoral fins which are enormously enlarged forming the 

 lateral expansions of the trunk; and a pair of pelvic fins 

 which are posterior to the pectoral fins and continuous with 

 them in some species. The pelvic fin will be found to con- 

 sist of two lobes, one of which in the males bears a long 

 clasper which is grooved on its posterior lateral margin 

 and is used in mating. Only a trace of the caudal fin will 

 be found at the end of the tail. For the normal fin develop- 

 ment see the shark on demonstration. 



3. Head. 



The head is very greatly flattened and its margins are 

 continuous with the pectoral fins. The head terminates in 

 a rostrum. A pair of eyes without lids is located on the 

 dorsal surface, and behind each eye is a spiracle or first 

 gill-slit. On the anterior face of the spiracle is a valve 



