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 branchiz, 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 shcell-tissue, 
and in some cases by pearl. 
A very beautiful example of this has been recorded 
by Dr. Giinther.t The specimen is represented in the 
accompanying woodcut (fig. 16). It had been in Dr. 
Giinther’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 /verasfer. 
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 
» “ Proceedings of the Zoological Society,” 1886, 
