MUSEUM OF COMPARATIVE ZOOLOGY. 231 



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 medusae 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 

 medusii'orm gonophores. In the development of the egg of Hydrichthys, it is 

 supposed that the planula, instead of fastening itself to some submarine oVjject, 

 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 sufi'ers a degeneration, or remains in a degraded 

 condition. 



The needs of procreation, however, still remain, and the necessity for the 

 locomotion of the sexual zooid 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 gunophoi'e 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 diff'erent 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 pojypite, 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 zoological position. As a question of opinion, the author 

 regards them as phylogenetically low, and nearer the ancestral form of hydroids 

 than Syneoryne and others. 



* Those who have studied the Hydromedusae 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. Hydrichtliys certainly sliows that this is true ; 

 for, if known from the hydroid alone, it might he placed in a zoological position 

 very far from that which its medusa would indicate as its true one. 



