FEEE MESSMATES. 39 



to procure for itself water, air, and food. The animal, 

 protected by the madrepore, can do without its calcareous 

 mantle, and only shows the end of the tube at the outside. 

 It is this organ which sustains the struggle against the 

 exuberant growth of the polyp, since it is by means of it 

 that the mollusc obtains nourishment. The Magilus is 

 like an oyster which is living in contact with a bank 

 of mussels, with this difference, that the oyster almost 

 always succumbs, while the magilus is always victorious 

 in the struggle. We might also cite as well as the 

 Magili, some Vermeti, certain CrepiduUe and Hipponices, 

 which struggle with the same success against those which 

 pilot or receive them. 



As there exist parasites which only depend on others 

 during their youth, so there are messmates which are 

 completely independent when fully grown. Jacobson, of 

 Copenhagen, wrote, in or about 1830, a memoir to show 

 that the young bivalves which are found in the external 

 branchial processes of the Anadont® are parasites, and he 

 proposed for them the name oi Glochidium. Blainville 

 and Dumeril were charged to make a report on this 

 memoir, which the author had sent to the Academie des 

 Sciences. But his opinion had not many supporters, 

 and it is now thoroughly known that the young anodonts 

 differ considerably in their early and their full-grown 

 state. During their stay in the branchial tubes, each 

 young animal carries a long cable which descends from 

 the middle of the foot, and serves to attach the anodont 

 to the body of a fish, and yet permits it to move to 

 a certain distance.* In fact the young anodonts have, 



* I owe this observation to Dr. W. S. Kent, who showed me, in London, 

 anodonts attached in this manner to sticklebacks. 



