CLASS AGNATHA 



361 



Figure 239. A lake trout showing a typical lamprey scar; arrow points to scar. This fish has 

 been the backbone of the Great Lakes fishing industry. (Courtesy of U.S. Fish and Wildlife 

 Service. ) 



mouth (Fig. 241) lies at the bottom of a 

 suctorial disk, the oral (buccal) funnel, and 

 is held open by a ring of cartilage. Around 

 the oral funnel are a number of papillae 



and horny "teeth." At the apex of the oral 

 funnel is the mouth, through which the 

 pistonlike tongue protrudes; it bears horny 

 teeth. On each side of the head is an eye, 



Figure 240. Female sea lamprey; a jawless vertebrate, 20% inches long, showing the charac- 

 teristically mottled back of a sexually mature adult. Note the laterally placed eye, behind 

 which are the gill slits. The sea lamprey is an eel-like parasite that preys on fish. (Courtesy of 

 Institute for Fisheries Research, Michigan Department of Conservation.) 



and posterior to the eye, 7 gill slits. Be- 

 tween the eyes on the dorsal surface is a 

 single opening, the nasal opening (Fig. 

 242). The anus opens on the ventral sur- 

 face near the posterior end; just behind it is 

 the urogenital opening in the end of a small 

 papilla. There are two dorsal fins and one 

 caudal fin (Fig. 240). 



Skeletal system 



The notochord of Petromyzon persists as 

 a well-developed structure in the adult (Fig. 

 242). In the trunk region it is supplemented 



by small cartilaginous neural arches. Cartila- 

 ginous rays hold the fins upright. The or- 

 gans in the head are supported by a car- 

 tilaginous cranium and a cartilaginous 

 branchial basket. 



Muscular system 



The muscles in the walls of the trunk and 

 tail are segmental, in a ^ -shaped arrange- 

 ment, similar to the fishes. The tongue is 

 moved by large retractor and smaller pro- 

 tractor muscles. The buccal funnel is oper- 

 ated by a number of radiating muscles. 



