38 ANIMAL PARASITES ' AND MESSMATES. 



The true relation between these molluscs and the 

 holothurians remains to be discovered, and how the 

 entoconchse become at last simple sexual tubes. At 

 present we must admit that it is the result of a retrogres- . 

 sive development like that of the peltogasters, which, like 

 them, lose all the attributes of their class. They ought, 

 perhaps, to be placed farther on, among parasites. 



Some years since, some molluscs were observed 

 which have compromised more or less the dignity of their 

 class. Graffe cites a species of the genus Cyprsea, which 

 one would certainly not expect to find in this category ; 

 it lives among the Viti Islands, in the compartments of 

 the Militheea ochracea. We have referred to it before. 

 Naturalists have given the name of Melithaea to a very 

 beautiful polyp which forms colonies of two or three 

 metres in height. Mons. Steenstrup, with that perspi- 

 cacity which discerns the most complex phenomena, has 

 also described Purpurse which live as messmates with the 

 Antipathes and the Madrepores. Quite recently, indeed, 

 •Mr. Stimpson has observed in the port of Charleston, a 

 gasteropod mollusc, similar to a Planorbis (fiochlicelepsis 

 parasitus) which lives as a messmate in the body of an 

 annelid (Occetes lupina). 



It is not the same with a mollusc called Magilus, 

 which naturalists considered for a long time to be the 

 calcareous tube of an annelid. All conchologists know 

 the shell of the Magili, so valued by collectors. This 

 gasteropod when young takes up its lodgings in the 

 substance of a madrepore which grows more quickly than 

 he, and in order not to die, stifled in this living wall, he 

 constructs a calcareous tube similar to the shell, of which 

 it appears to be the continuation, and which allows it 



