MUTUALISTS. 83 



mentioned, in his memoir on the habits of the Meloe, a 

 worm found in an egg. 



M. Barthelemy has studied a nematode worm (Asca- 

 roides limacis) which inhabits as a parasite the egg of the 

 grey snail; is this not the ordinary worm of the snail 

 which has introduced itself into the eggs ? 



Many animals establish themselves on their neigh- 

 bours, not to obtain any advantage from them, except to 

 profit by their fins ; they are not themselves sufficiently 

 adapted to rapid motion, so they seize a good courser, 

 mount on his back, and ask from him only a resting- 

 place and no provisions. But it is often very difficult 

 to say where commensalism ends and mutualism 

 begins; the cirrhipedes, for example, establish them- 

 selves on a piece of floating wood, or on the bottom of a 

 vessel ; on a block of stone, or on one of the piles of a 

 groin ; on an immovable animal as well as on a good 

 swimmer. 



Some fourteen years ago, Jacobson of Copenhagen 

 wrote an interesting essay, to show that the young 

 bivalves that are found in'the branchiae of anodonts at a 

 certain period of the year are parasitical animals, for 

 which he, proposed a new name. But these supposed 

 parasites are only young anodonts, which by the help of 

 a very long cable, which proceeds; from their foot like a 

 byssus, attach themselves to their mother, or to a fish 

 which will carry them to. a distance. 



We see full-grown acephalous molluscs, as mussels 

 and pinnae, still keep these cables, under the name of 

 byssus, during their whole life. There are also among 

 distomians, worms which though they are hermaphro- 

 dite, couple two and two, and have this additional pecu- 



