ENLARGEMENT OF PARTS FROM USE. 29 



fashion. The margins of the gills are covered with 

 cilia, which, by their constant movements, set up in- 

 halent currents, which not only serve to oxidise the 

 blood in the branchiae, but convey concrete particles, 

 many of which are seized upon by the mussel and 

 utilised as food. 



Some lamellibranchs have animals commensal upon 

 them. Commensalism differs from parasitism in the 

 important fact that an animal commensal on another 

 lives upon the food of its host, whereas a parasite lives 

 in the cavities or tissues of, and draws nourishment 

 from, the blood of its host. It would seem that as long 

 as the animals commensal on a lamellibranch keep 

 within the space between the mantle they are safe 

 enough, but occasionally they are rash enough to enter 

 the space between the shell and the mantle. This 

 trespass is resented by the lamellibranch, and the tres- 

 passer is punished by being entombed in shell-tissue, 

 and in some cases by pearl. 



A very beautiful example of this has been recorded 

 by Dr. Giinther. 1 The specimen is represented in the 

 accompanying woodcut (fig. 16). It had been in Dr. 

 Gtinther's possession for many years. It is an old shell 

 of Margarita margaritifera, in which there is embedded, 

 behind the impression of the attractor muscle, a perfect 

 individual of a fish belonging to the genus Fierasfer. 

 The fish is covered by a thin layer of pearl-substance, 

 through which not only the general outlines of the body, 

 but even the eye and mouth, can be seen. 



In this case the fish, instead of keeping between the 

 1 " Proceedings of the Zoological Society," 1886. 



