FOOD, HABITS, ETC. 83 
seized be a soft portion such as a fin or a gill-fila- 
ment, penetrate the tissues, and the irritation they 
set up causes an overgrowth of the skin of the fish to 
envelop the Glochidium, which thus cradled thrives 
for some two to six weeks on the juices of its host. 
Meanwhile the byssus and adductor muscle of the 
Glochidium disappear and the two permanent 
adductors of the adult and the foot are developed. 
The mantle secretes a new shell, resembling the 
adult form, beneath the Glochidium shell, which 
persists, however, for three or four weeks after the 
young Mussel is again set free by the perishing of 
the containing cyst. By the time, though, that it 
quits this temporary abode and proceeds to complete 
its growth, it has almost certainly been carried far 
from the place of its origin to some spot where 
perhaps there will be more room for it. 
Overcrowding is undoubtedly as bad for animals 
as it is for plants, and in the vegetable kingdom 
various well-known adaptations are provided to 
insure the dispersal of the seeds and their convey- 
ance to fresh soil. Among the Mollusca the instance 
just cited is perhaps the most distinct example of a 
like provision. With the marine Mollusca the 
shifting waters of the sea are practically the sole 
vehicle for the transportation of the young to new 
spots, and the embryos of nearly all, including those 
that, like the Oyster, are fixed in the adult state, are 
free swimmers, principally on the surface of the sea. 
