1885.] On a Parasitic Copepod of the Clam. IIQ 
added by Kellicott! and myself? to the list of those already de- 
scribed. 
That much remains to be done in this direction may be gath- 
ered from the common occurrence in the gill-tubes of the ordi- 
nary long clam (Mya arenaria) of an interesting form, so large as 
to occasion surprise that it has hitherto escaped notice. 
At the recent meeting of the Am. Assoc. for the Advancement 
of Science in Philadelphia, I proposed the generic name Myicola 
for the copepod in question, and shall describe the species as M. 
metisiensis, from the village of Little Metis, on the Gulf of St. 
Lawrence, where the specimens were taken. 
The order Copepoda affords most interesting material for the 
study of various grades of parasitism. At one end of the series 
are the completely free forms, at the other the completely para- 
sitic, in the adults of which it is often impossible to recognize any 
resemblance to the copepod type. But no important gap exists 
in the whole series. Even among those with well-developed jaws 
(Gnathostomata of Thorell) the Notodelphyide only occur as 
commensals in the branchial sacs of tunicates, while before we 
reach the completely parasitic forms, where the mouth is con- 
verted into a sucking tube (Siphonostomata),’we find a large 
number of forms (the Poecilostomata of Thorell), some free and 
some semiparasitic, where the parts of the mouth are evidently 
formed for piercing soft tissues and thus obtaining fluid nourish- 
ment. 
To the last section belong the Coryczidz, Sapphirinidz, Lich- 
omolgide, Ergasilide, to which groups some authors accord 
family rank, while others are disposed to regard them as consti- 
tuting a single family (Coryczide). It may be stated generally 
that while the fish parasites are chiefly Siphonostomata, and the 
ascidian commensals chiefly Notodelphyidz, the copepod para- 
sites of the other groups of the animal kingdom—Mollusca, 
Vermes, Echinodermata, Ccelenterata—are largely Poecilosto- 
mata. Thus members of this group have been found on the 
gills of cuttlefishes, on the delicate branchie of nudibranchs, in 
similar situations on marine annelids and holothuroids, and on 
the soft tissues of sea-pens and other ccelenterates. 
The clam parasite also belongs to the same group, as do all 
1 Proc. Am. Soc. Micros., II and Iv. 
2 Proc. Can. Inst., N. S. Vol. 1, p. 243. 
