FKEE MESSMATES. 17 



some specimens of these pinnotheres in the galleries of 

 the Natural History Museum at Paris. 



The large mussel, which furnishes fine pearls (Avicula 

 margaritifera), lodges also pinnotheres of a particular 

 species by the side of another messmate more allied to 

 a lobster than a crab. It is not even impossible that 

 these crustaceans, with other messmates or parasites, 

 contribute to the formation of pearls, since these gems, 

 so highly prized in the fashionable world, are only the 

 result of vitiated secretions, and are usually the result of 

 wounds. 



We also meet with a little crab (Ostracotheres tri- 

 dacnse, Euppel) in the acephalous mollusc, whose immense 

 shell sometimes serves as a vessel for holy water ; and it 

 lives doubtless in many other bivalves which have not 

 yet been examined. 



Dr. Leon Vaillant has written a very interesting 

 memoir on the TridacnaB, and informs us that the crab 

 takes shelter in their branchial chamber. Therefore, 

 since the molluscs live only on vegetable substances, 

 while the Ostracotheres feed entirely on animal matter, 

 Mons. Vaillant supposes that the latter take their choice „ 

 of the food as it enters, and seize on its passage that 

 which suits them best. Mr. Peters, during his abode 

 on the coast of Mozambique, studied a great many 

 of these acephala and pearl-mussels, and found their 

 interior inhabited by three crustacean decapods, a pin- 

 nothere, and two macrource allied to the Pontonia, to 

 which he has given the name of Conchodytes ; the 

 Conchodytes tridacnm inhabits the Tridacna squamosa ,- 

 the Conchodytes meleagrinse, as its specific name indicates, 

 lives in the shell of the pearl-mussel. 



