CHAPTER LI 



CLASS CYCLOSTOMATA 



The cyclostomes are sometimes separated from the other classes 

 of vertebrates to form a group higher in rank than a class and known 

 by the same name, Cyclostomata, or the name Agnatha. Such a group is 

 coordinate with Gnathostomata (nath o sto' ma ta; G., gnathos, jaw, and 

 stomatos, mouth), which includes all the other classes. The lack of jaws 



B 



Fig. 237. — Cyclostomes. A, California hagfish, Polistotrema stouti (Lockington) , with 

 12 gill slits. X 3'3- {Redrawn from Chidester, "Zoology," after Dean.) B, sea lamprey, 

 Petromyzon marinus Linnaeus, with seven gill slits. X 3^. From preserved specimen. 



and teeth, which are replaced by a suctorial mouth, is the most prominent 

 characteristic of the cyclostomes. They also have an elongated, eel-like 

 body with a leathery skin devoid of scales, are without paired appendages, 

 possess a fibrous skeleton to which in one type is added some cartilage, 

 have a single olfactory pit, usually an unpaired gonad, and a variable 

 number of gill slits. They have no air bladder, oviducts, or cloaca. 

 While free-living when young and feeding upon small particles in the 

 water, they are as adults generally parasitic on other fishes. 



359. Classification. — The cyclostomes are divided into two sub- 

 classes. The first of these, Myxinoidea (mik si noi' de a; G., myxinos, 

 sHme fish, and eidos, form), includes the hagfishes; the second, Petro- 

 myzontia (pet ro mi zon' ti a; G., 'petra, stone, and myzontos, sucked in), 

 contains forms known popularly as lampreys, or sometimes as lamprey 

 eels. The latter name, however, is not correct since the eels, properly 

 speaking, are bony fishes. 



360. Myxinoids. — ^The myxinoids, or hagfishes (Fig. 237^), are 

 noteworthy because of the enormous amount of slime they secrete when 

 captured and confined in a small space, one large specimen being said to 

 produce enough to fill a bucket. Hagfishes do not lead a strictly parasitic 



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