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 medusse 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 Chrysomitrae 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 stac^es 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 tlie wall of the fish is so firm that it is 

 with (lifTicully broken away. I was obliijed to cut it off. and with the hydroid 

 thus dissected portions of tlie body of the ho.«t were also n ptured. 



t There appears to be an opening at the free end /i tlie gonosome. I could 

 not determine to my satisfaction that the supposed opening really exists. I could 

 not observe that it was functional. 



