40 ANIMAL PARASITES AND MESSMATES. 



not like the other acephala, vibratory wheels in order to 

 move themselves ; they are conveyed in this manner by 

 their neighbours. There are also messmate acephala, as 

 the Modiolarla marmorata, which lodge on the mantle of 

 ascidians. Professor Semper found attached to the skin 

 of a Synapta simUis, a mollusc which possesses a pecu- 

 liarity rare among these animals, that of carrying its 

 shell in the interior and not on the outside. 



There are few animals so infested with parasites as 

 the Ascidians in general. Not only does their surface 

 sometimes become a microcosm, as the name of one Medi- 

 terranean species indicates, but even in the substance of 

 their testa lodge Crenelle and other molluscs and polyps, 

 which choose by preference to place their dwelling there. 

 There are also Annelids which hollow out galleries in 

 their interior, Lernseans which establish themselves in 

 their respiratory cavity. Nematodes, Pycnogonida?, 

 Ophiurae, and many others besides. Mons. Alfred Giard 

 has described several Amphipods and Isopods which 

 establish themselves on Tunicates. One cannot say 

 that there is always such a complete agreement between 

 animals of such different kinds, for Mons. Alfred Giard 

 gives examples of grave disagreements which he has 

 seen break out, and which have caused the death of 

 several among them. 



Another association is that of a gasteropod with one 

 of the acephala. In the environs of Caracas lives an 

 Ampullaria {Crocostoma) which lodges in the umbilicus 

 of its shell another mollusc, the only fluviatile species 

 of those countries, called the Sphaerkim modioUforme. 

 We have every reason to suppose that the Sphaerium 

 lives on good terms with the Ampullaria, since they are 

 usually found associated. 



