MUSEUM OF COMPARATIVE ZOOLOGY. Zoi, 
Sarsia shows that the affinities of the genus are higher than its hydroid would 
seem to indicate. There is a pretty close likeness between most of the hy- 
droids of the meduse to which the Hydrichthys medusa is allied. This radical 
departure in Hydrichthys in the form of the hydroid itself may have a mean- 
ing, and the exceptional anatomy is thought to be due, at least in part, to its 
parasitic life, especially as the medusa is so closely allied to other tubularian 
medusiform gonophores. In the development of the egg of Hydrichthys, it is 
supposed that the planula, instead of fastening itself to some submarine object, 
becomes attached to the fish. The necessities for the development of tentacles 
would be reduced from the fact that the fish (Seriola) carries the hydroid 
about, and perhaps furnishes sustenance for the parasite from its own body. 
As a result, the hydroid suffers a degeneration, or remains in a degraded 
condition. 
The needs of procreation, however, still remain, and the necessity for the 
locomotion of the sexual zodid and the organs for the development of new 
individuals is in no way diminished by the parasitic life of the hydroid. In- 
stead of being reduced in size, they are, if anything, enlarged in number ; and 
as the medusiform gonophore separated from the gonosome is placed under 
exactly the same conditions as that of any fixed hydroid, it retains characters 
of its near relatives.* ; 
Hydrichthys has certain features in the anatomy which recall the floating 
hydroid, Velella. The gonosomes resemble in several particulars the sexual 
bodies of Velella, and the free medusa is not very different from Chrysomitra, 
the medusa of Velella. The flat basal disk also of Hydrichthys has points of 
resemblance to the basal plate and the ramifying tubes on the under side of 
the float in the well-known V. spirans. In the polymorphism of the two there 
is some likeness. In Velella we have a single non-tentaculated “central 
polyp,” or polypite, surrounded by many sexual bodies, or gonosomes. We 
have in Velella, moreover, two kinds of individuals, which is perhaps the 
simplest kind of polymorphism anywhere known among Siphonophores, 
except in the kindred genus Porpita. In Hydrichthys we also have two 
ever have a free medusiform gonophore. They probably have a development 
like Hydra, and are destitute of special locomotive zooids. 
It is, of course, an open question whether Hydra and Protohydra are nearer the 
ancestral type than other hydroids. It is not unlikely that they are degenerate 
forms, and not ancestral. The peculiarities of their habitat in fresh water might 
have led to their low zodlogical position. As a question of opinion, the author 
regards them as phylogenetically low, and nearer the ancestral form of hydroids 
than Syncoryne and others. 
* Those who have studied the Hydromeduse have for the most part based 
their classifications either on the form of the hydroid, or the form of the medusi- 
form gonophore. Both are in error if they rely upon either hydroid or medusa 
alone as a basis of classification. Hydrichthys certainly shows that this is true ; 
for, if known from the hydroid alone, it might be placed in a zoological position 
very far from that which its medusa would indicate as its true one. 
