154 ESSENTIALS OF ZOOLOGY 



fins on the body and tail, and these vary in position and number 

 according to the fish. The median fins are resolved into 

 dorsal, caudal, and ventral. The ventral fin, or fins, is some- 

 times and unfortunately called the anal fin or fins, because it 

 or the first of the ventral fins lies behind the anus, and the 

 name ventral is given by the same writers to the pelvic fins. 



The skate, in association with a leisurely life on the mud 

 and sand at the bottom of the sea, has become flattened. 

 The tail has lost its locomotory power, and the pectoral fins 

 have been greatly expanded with their muscles to act as 

 propelling organs. During its early development in the egg, 

 however, the skate is like other fish and is actually laterally 

 compressed. The change, as will be observed from fig. 75, 

 is initiated by the expansion laterally of the paired fins and 

 especially of the pectoral fins, and this is followed by a flattening 

 of the body and head. 



Skates and rays feed principally on Crustacea and fishes. 

 They are found in all seas, and they have been procured 

 also from great depths, 500 to 600 fathoms. They are 

 common in moderate depths, and are landed by fishermen at 

 most ports. The common grey skate of British seas is Raia 

 batis, and it attains a great size and weight. The smaller 

 rays, as Raia clavaia, the thornback, R. radiata, the starry 

 ray, are also landed in large numbers, and such are more 

 useful for laboratory work. The attempt should be made 

 with the aid of systematic works on fishes to identify the 

 species, but the identification is not always easy, for rays are 

 liable to a great deal of variation and the distinction between 

 them is sometimes very slight. Fundamentally and essen- 

 tially they are all the same in structure, and the following 

 description will apply to them all, and even allow of the 

 dissection of their allies, the dogfish, being followed. 



Skates are sometimes obtained bearing the large external 

 parasite, the skate leech, Pontobdella muricata ; it belongs to 

 the class Hirudinea. In the gills the trematode parasite 

 Onchocotyle appendiculata is sometimes observed, and parasitic 

 copepod crustaceans may be found in the nasal sacs. 



EXTERNAL CHARACTERS. The body is flattened and rhom- 

 boidal in shape. The tail is long and narrow and leaves the 



