﻿140 



PROCEEDINGS OP THE 



[April, 1857. 



descending to a marginal sinus. The sexual organs are situated 

 as usual among Endostomata, around the digestive trunk. There 

 appears to be no distinctly circumscribed ocellus, and the (origi- 

 nally two?) short tentacula are terminated by a wheel-shaped 

 appendage, which is attached, not by its circumference, but by 

 its centre. The genus presents also the remarkable peculiarity 

 (so far as I know, not observed in any other free Hydroid Medusa, 

 unless it be Cladonema,) of having the inner surface of the disk, 

 between the radiate tubes, conspicuously colored. A reference 

 to the description of the Medusa-buds, in the following species of 

 Porpita, will show that there is some probability that this is a 

 family character, or at least not uncommon in the group. It has 

 another character which is common to a large number of genera 

 in the next group — that of Tubularidae. It is the existence of 

 rows of thread-cells, extending upward along the outer surface of 

 the disk, from the margin towards the summit; a localization of 

 these organs very different from their scattered state in the Coryni- 

 dae. It is something analogous to the localization of the pedicel- 

 lariag into belts (fasciolas) among Spatangoids. 



The larvae of Velellidae are free floating oceanic hydroids. We 

 should consider each of them as a community of animals, which 

 in some respects is analogous to the community formed by a sin- 

 gle polyp-head of Tubularia, with its numerous pedunculated 

 Medusae. There is a central fibrous shield, consisting of numer- 

 ous concentric air canals, which may be taken as the repre- 

 sentative of the horny polypidomata of fixed genera. Around 

 the periphery of this shield the general fleshy mass is produced 

 in a sort of border. Immediately beneath this border radiate on 

 every side, a number of organs which might be compared in posi- 

 tion to the lower circle of tentacula in Tubularia or Pennaria. 

 But it is doubtful whether, finally, we shall not be obliged to con- 

 sider these as each an individual of a peculiar form. The centre 

 of the lower surface of the disk is occupied by a large, broad- 

 based polyp, of the form of an inverted cone; from its base pass 

 towards the periphery, a sort of canals which communicate with 

 the canals of the outer circle of the so-called tentacula, and with 

 the cavities of the individuals next to be described. These are 

 untentaculated polyps, which seem nevertheless to be provided 

 each with a mouth and a digestive cavity. They should be com- 

 pared to the ramified stem bearing the Medusae in Tubularia, to 

 which they are analogous in position. Each of these, however, 



