SEA-FISHERTES LABORATORY. 369 
is probably in some exceptional condition. In addition 
to these features the cells have long cilia-like processes 
which are almost as long as the cells themselves, and 
these meet in the centre of the space. In the Pearl 
Oyster (Margaritifera vulgaris) (2). where there is a well- 
developed byssus, the same threads are present in 
connection with the secreting cells of the byssus, and they 
pass into the horny mass which appears to have been 
secreted round them. No byssus is present in P. maximus, 
but these cells are presumably the gland cells, and the 
pecuhar cilia-lke processes are identical with those 
forming the root of the byssus in the Pearl Oyster. 
With regard to the functions of the foot, which seems 
so rudimentary in the adult, we have already seen that 
in the early life history it functions as a locomotive organ, 
while later on it becomes an organ for attaching the 
byssus threads to a foreign body. In the adult it seems 
from observations to perform neither of these functions, 
though in the case of P. opercularis, the time during 
which byssal threads are formed is probably long, and in 
P. varius the byssus is spun and fixed even in the adult 
stages. 
That the foot can be moved about and extended is 
easily seen when watching the living animal, and in one 
case 1t was observed that this extension brought the apex 
of the foot outside the valves altogether. It probably is 
of use, therefore, for freeing the palps and gills of foreign 
particles, as the foot of the pearl oyster has been observed 
to do. The cavity at the apex always contains a quantity 
of mucus, and we have already seen how well the foot is 
provided with glands. This mucus may be secreted for 
the purpose of entangling the food particles that are 
wafted to the mouth, by the gills and palps. 
