ON A FREE-SWIMMING HYDROID. 21 



be closely related to Corymorpha, all four tentacles are 

 developed. 



It is a very curious fact that two distinct genera of 

 Tubularian hydroids agreeing in such striking anatomical 

 peculiarities should have become adapted to two such differ- 

 ent modes of life, the one swimming freely in the open ocean, 

 and the other rooting itself in the sand at the bottom. It 

 would indeed be difficult to find a better example of the 

 powers of adaptation to divers conditions of life. So far as 

 I am aware there is no other hydroid yet known which has 

 become specially adapted to a pelagic mode of life. It is 

 true that floating hydranths — Acaulis and Ne mop sis — 

 are known, but these have probably become detached from 

 stalks, and are not structurally adapted to a free-swimming 

 existence. 



6. Diagnosis op New Genus and Family. 



Genus Pelagohydra, n. gen. — Hydroid solitary, free- 

 swimming; the proximal portion of the body modified to 

 form a float, supported internally by a system of radiating 

 membranes of endodermal origin ; the distal portion forming a 

 flexible proboscis, with the mouth at its extremity. Gastral 

 cavity continued from the proboscis into the float in the form 

 of endodermal canals, from which arise branching stolons. 

 Tentacles filiform, scattered over the surface of the float and 

 in whorls around the mouth. Medusae developed on stolons 

 between the tentacles of the float; quadriradiate, symmetri- 

 cal, probably with gonads in the wall of the simple manu- 

 brium ; tentacles in four per-radial groups of five (possibly 

 more in the adult). 



The genus may be regarded as belonging to a distinct 

 family, for which I propose the name Pelagohydridas, and 

 for which the generic diagnosis may at present suffice. This 

 family is, however, closely related to the "Corymorphinje" 

 of Delage and Herouard;^ indeed, some zoologists might 

 > * Traite de Zoologie concrete : ' " Les Coelenteres," p. 88. 



