CHAPTER III 



CYCLOSTOMES 



Cyclostomes are the most primitive vertebrates, in many re- 

 spects intermediate between the Amphioxi and the fish. They 

 are popularly called "round mouthed eels", the name being 

 derived from a superficial eel-like structure, not from any 

 anatomical similarity. The animals inhabit both fresh and salt 

 water and are very widely distributed. 

 The characteristics of the group are: 



1. No paired appendages. The cyclostomes entirely lack the 

 two pairs of appendages typical of the vertebrates. The student 

 should not confuse the leglessness of snakes, some fish, and other 

 animals with the inherent absence of paired fins in this class of 

 vertebrates. In the higher legless forms there are always 

 embryonic limb buds, which may remain as rudiments in the 

 adult. The cyclostomes have no embryonic evidence of ever 

 having had fins. 



2. Round mouth. The word cyclostome means ''round mouth", 

 in reference to the round oral hood, somewhat similar to that 

 of Amphioxus. A rasping "tongue" scrapes the food into fine 

 fragments, while the sucking oral hood attaches the animal to 

 its prey. 



3. No jaws. Lacking jaws, the oral hood remains permanently 

 open, the two characters being interdependent. The absence of 

 jaws is a very primitive characteristic, as is shown by both 

 embryological and fossil evidence. 



4. Gill structure. One of the orders of living cyclostomes has 

 seven gill openings, the other has as many as fourteen. As five 

 gill slits is the typical number of the higher vertebrates, and 

 the most primitive fish known has seven, the cyclostome condi- 

 tion is considered primitive. The internal and external openings 



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