CHAPTER XXV 



CYCLOSTOMATA* 



Because of the absence of jaws this group is sometimes known 

 as Agnathostomata (ag nath o sto' ma ta). This name is in contrast 

 to Gnathostomata (jaw mouth) which includes all other vertebrates. 

 The mouth of the cyclostomes is round, jawless, and suctorial. There 

 are some exoskeletal teeth located on the roof and floor of the 

 mouth and on the tongue. The body is slender and eel-like in shape. 

 It is covered with a slippery, smooth skin and has only dorsal and 

 ventral median fins. 



Classification 



The group is divided into two subclasses (or orders according to 

 some authors) distinguishable by presence or absence of tentacles 

 around the mouth, number of gill slits, and the number of semi- 

 circular canals. These subclasses are Myxinoidea (Hyperotreti) in- 

 cluding the hagfishes; and Petromyzontia (Hyperoartii) including 

 lamprey (or improperly, lamprey eel to some). 



Myxinoidea or hagfish are all included in one family Myxinidae 

 which is divided into three genera : Myxine of the Atlantic and Pacific 

 Oceans, Bdellostoma and Paramyxine of the Pacific. These each 

 have specific characteristics, but they all agree in having a terminal 

 nostril, four tentacles on each side of the mouth, ability to produce 

 enormous quantities of mucus, and the lack of the oral funnel or 

 sucker. They all possess twelve or more pairs of gills, only one 

 semicircular canal in the inner ear, and a functional pronephros. 

 The development of the hagfish does not include a metamorphosis. 

 They usually live in the mud of the sea bottom except when they 

 are feeding either on the dead body of a fish or attached to a live 

 one. They frequently enter the mouth or gills of fish caught in 

 nets or those dead from natural causes and devour all of the inter- 



•If the frog or toad Is to be used as the laboratory animal representing the typical 

 vertebrate, and the instructor so desires. Chapters XXV, XXVI, and XXVII may be 

 omitted until after the study of Chapter XXVIII and then assigned if time permits. 



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