82 ANIMAL PARASITES AND MESSMATES, 



our coasts (Cancer moenas) a nemertian wliich probably 

 performs the same office. He is lodged while young in 

 a kind of firm sheath attached to the abdominal pro- 

 cesses. We have been able easily to study the first 

 phases of its evolution. We have given it the name of 

 Polia involuta. 



This nemertian had been observed at Messina, and 

 described before by Kolliker under the name of Nemertes 

 carcinoj)hilus, and it has just been described and figured 

 anew by Mr. M'Intosh, in a monograph of British an- 

 nelids published by the Bay Society. 



The sturgeon seems to give lodging in its eggs to a 

 polyp which plays the same part. In fact, Mons. Ows- 

 jannikoif, at the congress of Eussian naturalists at Kiew, 

 described an animal, Accipenser ruthenus, which lives in 

 the eggs of the sterlet. Some eggs placed in water for a 

 few hours at first show tentacles on the outside, then a 

 whole colony, and each part consists of four individuals, 

 which have a common digestive cavity, resembling some- 

 what a hydra divided longitudinally in four. Each has 

 six tentacles, two of which are terminated by transparent 

 corpuscles, perhaps nematocysts ; the digestive cavity 

 extends into the arms, as in the hydra ; the mouth is 

 not between the tentacles, but at the oiDposite pole. 

 They are not all lodged within the eggs ; some are found 

 outside, according to the observations of Mons. Koch. 

 Does not this animal fulfil in the egg of the sterlet, 

 the same office as the histriobdella in the egg of the 

 lobster 7 



The eggs of some insects are attacked by very little 

 ichneumons, the Proctotriqndw ; they empty them, and 

 then instal themselves in the shell. Mons. Fabre has 



