1894.] Hydroid : Cold Spring Harbor, Long Island. 29 
[Bull. Mus. Comp. Zool., Vol. XIII, p. 224]. In this case how- 
ever, it was not certain that the parasite did any damage to 
the fish. It is not impossible that a hydroid could get into 
such a relation to the soft tissues of a crab at the joints as to be 
a harmful resident there, but we know of no proof that such 
is true of Perigonimus jonesii. 
An examination of the colony with the low power shows that 
it is a very much branched mass of stalks arising from stolons 
that ramify on the surfaces offered by the shelly outer cover- 
ing of the host. The colony is, unlike most of the other species 
of this genus, very closely and luxuriantly branched, and the 
two kinds of persons are both carried on the same stalk. The 
stalks are terminated with one oldest zooid and below it are 
the younger ones and the gonozooids. The hydriform per- 
sons or hydrozoids are, as shown in figure 2, only slightly differ- 
entiated into a body separate from the stem and in this respect 
it is of the more primitive hydroid form. The tentacles are not 
numerically constant but are about sixteen in number. They 
tend, when fully expanded, to assume an alternately reflexed 
position. They are confined to a single row at the base of the 
proboscis. This is also a primitive character. The stem just 
at the base.of the body proper of the hydrozooid bears a little 
bud not sufficiently developed to project beyond the outline of . 
the cuticle. The interesting thing about this hydroid to the 
biologist is the very primitive condition of the external skele- 
ton. This is a gelatinous investment that covers the stem and 
the body in every part. It is so soft and flexible that it follows 
all the movements of the body and stem without any apparent 
resistance. This cuticle reaches upon the body to the level of 
the tentacles to which it is fastened, as is shown by the fact that 
it is moved about with them as the animal waves them to and 
fro, and it is retracted with them as the proboscis is drawn 
