226. BULLETIN OF THE 
any inconvenience from the attached parasite. The muscles of the fish, how- 
ever, under the “ basal plate” of the hydroid were somewhat wasted ; and after 
the fish was killed, the shrinkage in its body walls seemed to indicate that the 
fish had not wholly enjoyed his strange companion. 
Hydroid. — The hydroid colony of Hydrichthys forms a cluster of reddish 
and orange-colored bodies attached to the sides and circumanal region of 
Seriola zonata. 
The base of the whole colony is about three fourths of an inch in lateral 
extent. The base of attachment to the fish is a flat thin plate with ramifying 
tubes, by means of which the colony is fastened to the fish, and upon it separate 
clusters of sexual bodies (gonosomes) and filiform structures (hydranths ?) are 
united together. The structure of the flat plate is not peculiar to Hydrich- 
thys, but resembles that of many other hydroids attached to submarine objects, 
as Perigonimus and Hydractinia. The walls of the basal plate are leathery, 
or coreaceous, rather than calcareous. This basal plate is destitute of promi- 
nent projections such as exist in Hydractinia, but is smooth both above and 
below.* . 
In studying the character of the basal plate of Hydrichthys I was reminded 
of the anastomosing tubes on the under side of the float of Velella. 
Each gonosome (Plate IV. Fig. 2) is botryoidal, consisting of an axis and 
lateral branches with medusz in all stages of growth. The axis of the gono- 
some arises by a single trunk from the basal plate, and tapers uniformly from 
attachment to apex, opening t at the free end into the surrounding water. 
This axis resembles in its histological structure the stalk which bears the 
medusiform gonophores or Chrysomitre in the genus Velella. It is sensitive, 
highly contractile when touched, transparent, or but slightly colored. 
The side branches are similar in structure to the stem. They are generally 
simple, but sometimes subdivided or branched. The lateral branches near the 
base of attachment are longer than those near the free end of the stem. The 
side branches are of uniform diameter, and arise irregularly from the main 
stem. Like the main stalk, they have a cavity within, which communicates 
freely with that of the main stem. 
In specimens preserved in alcohol the lateral branches are short and con- 
tracted, but in live specimens both the main stalk and its lateral branches are 
long and extended. There is no chitinous sheath about the axis or branches. 
Each lateral branch or supplementary division bears at its free extremity, 
which is closed, a cluster of medusa buds in all stages of growth from a simple | 
spherical enlargement or expansion of the axis to a medusiform body with two 
* The attachment of the basal plate to the wall of the fish is so firm that it is 
with difficulty broken away. I was obliged to cut it off, and with the hydroid 
thus dissected portions of the body of the host were also ruptured. 
{+ There appears to be an opening at the free end of the gonosome. I could 
not determine to my satisfaction that the supposed opening really exists. I could 
not observe that it was functional. 
