﻿168 



PROCEEDINGS OF THE 



[April, 1857. 



Ostend, I suggest, are males and females of one species. I have 

 found the same differences of habit and in the medusa-buds, be- 

 tween those of Charleston Harbor. 



Remark. — The true position of Hydractinia is a question of 

 some difficulty. Its affinities are complex, and probably indicate 

 that it should constitute a minor group of itself. Its tentaculated 

 polyps are like those of the larvae of Hippocrene, from which it 

 again differs in its horizontally expanding polypidom. On the 

 other hand, its medusa-bearing polyps are untentaculated like 

 those of the Velellidse. For this reason I am disposed to think 

 that it will constitute, when better known, an under-group in the 

 neighborhood of Velellidse. 



Distribution. — England, Denmark, Holland, France, Long Isl- 

 and Sound, Charleston Harbor. 



HYDRACTINIA ECHINATA. Johnston. 



It is impossible to distinguish, with any certainty, this species 

 from the European. There appears to me to be a greater delicacy 

 and slenderness in the general form of the polyps. The tentacula 

 appear to be more numerous, and I think also the spines, espe- 

 cially in the female polypidom, more marked and longer. But 

 this comparison is made only from drawings. 



I think there can be no doubt that this species is the same as that 

 observed by Leidy at Point Ju dithj nj^w.Jersey. Has Hydrac- 

 tinia been observed in Massachusetts? 



According to my observations, embryos are never found in the 

 form which encrusts the shells (especially of Natica) inhabited 

 by Pagurus. They have a form of medusa, also, in whose sex- 

 ual organ no ova are seen, but which contain a much more promi- 

 nent diverticulum of the polyp's canal, than does the egg-bearing, 

 embryo-bearing medusa of the other form, which encrusts rocks. 

 The latter form exists in countless thousands of individuals, covering 

 yards of surface of rock, while the former are confined to the outer 

 surface of shells scarcely more than an inch or an inch and a half 

 in diameter, and are comparatively rarely met with. If these are, 

 as I suppose them, male and female communities of the same spe- 

 cies, it will explain the reason why, while the multitudinous 

 females are stationary, the males, few in number, attach them- 

 selves to the shells of Paguri, whose nomadic habits thus insure 

 a wider distribution of the seminal product. 



