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bulletin of the bureau of fisheries. 
crab lives in the gill chamber, where it is protected from outside harm. In return for 
this protection it is said that the crab runs out and collects food which on returning it 
chews up in the gill chamber and shares with its host. From the examination of the 
stomach contents of several of these crabs, however, I found no evidence to support this 
belief. The only food material found consisted of diatoms and other microscopic 
organisms which probably would have been utilized by the mussel had not the parasite 
been present. Other hosts, such as the giant scallop and smooth scallop, are known to 
harbor this same species of crab. In describing it (Goode, 1884), Rathbun says: 
Another species of Pinnotheres ( P . maculatum ) frequently occurs in the shells of the common sea 
mussel ( Mytilus edulis) and the smooth scallop ( Pecten tenuico status), between the gills of the animal. 
It attains a size larger than the oyster crab, and, as in the case of the latter, the females alone are para- 
sitic, the males only having been found swimming at the surface of the sea. We have never heard of this 
species being eaten, probably because neither the mussel nor the smooth scallop has ever been used as 
food in this country. In the summer of 1880, while dredging off Newport, R. I., the United States 
Fish Commission steamer Fish Hawk came upon extensive beds of the smooth scallop, from a bushel 
of which nearly a pint of these crabs were obtained. Again, in 1881, the same species was encountered 
in great abundance by the same party in Vineyard Sound, in Mytilus edulis. As an experiment, they 
were cooked along with the mussels and found to be very palatable, although their shell is, perhaps, 
somewhat harder than that of Pinnotheres ostreum. 
In my own experience with mussels I have observed no other parasite, but in 
Europe Lebour (1907) found a boring annelid, Polydora ciliata, which attacks the 
Northumberland mussels. The worm burrows through the shell from the outside, 
making a hole about the size of a pin. It causes the mussel to grow pearly excresences, 
often to considerable extent, over the internal surface of the shell, which interfere with 
the muscular development of the animal and frequently almost destroys the posterior 
adductor muscle. If the pearly masses press upon the mantle, the reproductive lobes 
fail to develop in such places. Aside from injuring the mussel, the presence of the 
pearly excrescences gives the mussel an unsightly appearance and consequently renders 
it unfit for market. 
Three larval trematodes are also found in the Northumberland mussels. The 
cercarias of the pearl trematode have been found in the mantle, the encysted cercarias 
of Echinostomum secundum in the foot, and a third unidentified species encysted in the 
liver. These trematodes, however, even when present in large numbers, work very 
little injury to their host. 
Several species of mollusks are commonly found living with the sea mussel. Oysters 
are very often associated in the same beds with them and usually to the detriment of 
the oyster, if the mussels are present in large numbers. The mussels, having the power 
of free movement which the oysters do not possess, are able to acquire the more favorable 
positions for collecting food and thus deprive the oysters of much nourishment. The 
soft-shelled clam, Mya arenaria, and the hard-shelled clam, Venus mercenaria, are 
sometimes found growing among the mussels in good, healthy condition. Boat shells, 
Crepidula fornicata and C. convexa, are very common. Sometimes three or four individ- 
uals are attached to a single mussel, covering it almost completely, but apparently doing 
