62 ANIMAL PARASITES AND MESSMATES. 



One of these Halodactyles spreads itself upon the stalk 

 of a Sertularia, all the inhabitants of which it stifles, so 

 that it is the victim himself who serves as a guardian to 

 the invader. 



These Halodactyli are very widely spread over the 

 Northern Seas, and often establish themselves on the 

 large horse-hoof oyster. Michelin has noticed under 

 the name of parasite a fossil cellepore from the saltpits of 

 Touraine and Anjou, which entirely surrounds the shell 

 of a gasteropod ; in order to prevent its patron from dying 

 of hunger, the bryozoon develops itself around the mouth 

 like a gallery, and prolongs its last spiral. This Celle- 

 pora parasitica has evidently a place here. 



Many of these messmate bryozoa are found in a fossil 

 state in the crag of the Antwerp basin. 



We have still to mention among fixed messmates 

 many polyps, some of which are very remarkable. Thus, 

 many naturalists speak of vast colonies of polyps in 

 which lodge various animals which shelter themselves 

 there like paguri in deserted shells. 



Among these are the colonies of which Porster 

 speaks, which are not less than three feet in diameter, 

 and fifteen feet in height, with a crown of eighteen feet 

 in diameter. Dana also makes mention of an Astrsea 

 of twelve feet in height, and of Porites twenty feet high, 

 which contain more than five millions of individuals, 

 among which a number of animals come to take refuge. 



The Museum of Natural History at Paris is in 

 possession of a superb specimen of Porites conglomerata : 

 in the middle of the colony lodges a Tridacna (Trid. 

 corallicola, Val.) like a pagurus under a forest of 

 hydractinise. This remarkable polyp was brought from 



