GILL-OPENINGS. 39 



in order to more effectually close the gill-opening ; and this 

 fringe is supported by one or several or many bony rays, the 

 hranchiostcgals. The space on the chest between the two 

 rami of the lower jaw and between the gill-openings is called 

 the isthmus. 



The Sharks and Eays differ from the Teleosteous and 

 Ganoid fishes in having five branchial slits (six or seven in 

 Hexanchus and Heptanchus), which are lateral in the Sharks, 

 and at the lower surface of the head in the Rays (Fig. 1, p. 34). 

 In Myxine only the gill-opening is at a great distance from 



Fig. 2. — Head of Mordada mordax, showing the single 

 nostril, and seven branchial openings. 



the head ; it is either single in this family (Cyclostomi), or 

 there are six and more on each side (Tig. 2). 



In the Trunk are distinguished the hack, the sides, and the 

 abdomen. It gradually passes in all fishes into the Tail ; Tail, 

 the termination of the abdominal cavity and the commence- 

 ment of the tail being generally indicated by the position 

 of the vent. The exceptions are numerous : not only certain 

 abdominal organs, like the sexual, may extend to between the 

 muscles of the tail, but the intestinal tract itself may pass far 

 backwards, or, singularly, it may be reflected forwards, so 

 that the position of the vent may be either close to the 

 extremity of the tail or to the foremost part of the trunk. 



In many fishes the greater part of the tail is surrounded 

 by the fins, leaving only a small portion (between dorsal, 

 caudal, and anal fins) finless ; tliis part is called the free por- 

 tion or the peduncle of the tail. 



