342 



METAZOAN PHYLA 



life, since they commonly enter the gills or mouths of dead fishes and 

 remain in the body, feeding upon it, until they have completely destroyed 

 all but the skin and skeleton. They also may attack living fish, if these 

 are disabled. Marked characteristics are the possession of four pairs 

 of tentacles around the mouth, the presence of but one semicircular 

 canal in the inner ear, a variable number of gill slits, and a pronephros, as 

 well as a mesonephros, in the adult. There is no metamorphosis in the 

 hagfishes. They are hermaphroditic. 



361. Lampreys. — The mouth of the lampreys (Fig. 2375) is provided 

 with an oral funnel armed with chitinous teeth (Fig. 238). Cartilages 

 are present in the skull, about the notochord, around the gills, and in 

 connection with the fins, but the notochord remains in full development. 

 The number of gill slits is usually seven. The functional kidney in the 

 adult is a mesonephros. The ureter opens into a urinogenital sinus. 



Notochord 



Dorsal aorfa 



Nasal opening 

 Olfactory sac 



Dorsal cartilages 



■Annular cartilage 



Buccal 

 cavity with 

 chitinous teeth 



Mouth 



Internal \ 



openings of Esophagus 

 gill slits 



Fig. 238. — Median longitudinal section of the anterior part of the body of an adult sea 



lamprey. From a specimen. X /'4. 



Tongue 

 cartilage 



Annular 

 ccirfilage 



which also receives the sex cells liberated in the body cavity from the 

 gonad. The brain is primitive, in many respects resembles that of the 

 embryos of higher forms, and has a very small cerebellum (Fig. 239). 

 The spinal cord is much flattened dorsoventrally. There are two semi- 

 circular canals in the ear. 



Lampreys live in both fresh and salt water and are active and pre- 

 daceous, attacking fish much larger than themselves. They attach 

 themselves to their prey by means of the sucker-like oral funnel, which 

 is prevented from slipping by the chitinous teeth it contains. The 

 flesh of the fish is then lacerated with the sharp chitinous end of the 

 tongue so that blood and lymph can be sucked from the body. When 

 the body of the fish has been drained of its fluids or the lamprey is filled, 

 the latter loosens its hold, and the former goes away with an open sore 

 which usually becomes infected and results in its death. Lampreys 

 are, therefore, serious enemies of fish. 



The larva, called an ammocoetes, has an oral hood something like 

 that of amphioxus, a median eye, an endostyle, also resembling that of 



