CHAPTER 24 



Class Agnatlia 



(lawless Vertebrates). 



Lampreys and 



Hagfishes 



HE animals in the order Cyclostomata 

 (Fig. 240) represent a primitive level of 

 vertebrate development. The name means 

 round mouth. These eel-shaped vertebrates, 

 without jaws or paired fins, and with only 

 one olfactory pit, are commonly known as 

 lampreys, hagfishes, and slime eels. 



The suborder M^'xinoidea, or hagfishes, 

 are all marine; and the suborder Petromy- 

 zontia, or lampreys, occur in both salt water 

 and fresh water. The cyclostomes usually 

 feed on the blood and tissue fluid of fishes 

 which they attack with their rasping mouth 

 (Fig. 239). The cyclostomes are not only 

 interesting chordates, but some are of great 

 economic importance. 



SEA LAMPREY 



Petromyzon marinuSy the sea lamprey 

 (Fig. 240), a rather unpleasant animal, is a 

 modified survivor of some of the first verte- 

 brates that lived on the earth. It inhabits 

 the waters along the Atlantic Coast of North 

 America, Great Lakes, the coasts of Europe, 

 and the west coast of Africa. It swims about 

 near the bottom by undulations of its body, 

 or, when in a strong current, progresses by 

 darting suddenly forward and attaching it- 

 self to a rock by means of its suctorial 

 mouth. In the spring adult lampreys ascend 

 the rivers to spawn. 



External anatomy 



The marine lamprey reaches a length of 

 about three feet. Land-locked populations 

 such as those in the Great Lakes attain a 

 maximum size of only two feet. The body 

 of the lamprey is nearly cylindrical, except 

 at the posterior end where it is laterally 

 compressed. There is no exoskeleton. The 

 skin is soft and slimy, made so by secretions 

 from epidermal glands. It is a mottled 

 greenish-brown in color. A row of segmental 

 sense pits, the lateral line, is located on 

 each side of the bodv and on the head. The 



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