26 ON THE AFFINITIES OF THE FAMILY OF THE MEDUSA 



64. The adult forms of these organs have all the same structure, 

 being composed of two membranes, with a vast number of thread-cells 

 of larcrer or smaller size, seated in the substance of the outer membrane 

 or between the inner and the outer. 



65. The " clavate organs " of Plumitlaria deserve especial notice, as 

 I am not aware that they have been hitherto described, and as they 

 exemplify in a very beautiful manner the "unity of organization" 

 manifest among these families. 



I have found them in two species of Plumiilaria obtained by the 

 dredge at Port Curtis ; they were of two kinds, the one attached to the 

 cell of the polype, the other to the pedicle of the ovary, figs. 43, 44, 45. 

 In each species there were three processes of the former kind, two 

 above proceeding from near that edge of the aperture which is towards 

 the stem, the other below from the front part of the base of the cell ; 

 they were conical in the one species, club-shaped and articulated in 

 the other, and consisted of an external horny membrane open at the 

 apex, and an internal delicate membrane inclosing a cavity, all these 

 being continuous with the corresponding parts of the stem. At the 

 apex of each, and capable of being pressed through the aperture, lay a 

 number of thread-cells ; with moderate pressure the threads only of 

 these organs were pressed out. 



I found the second kind of organ in the species with conical pro- 

 cesses. It consisted of a stem proceeding from the pedicle of the 

 ovary, bearing a series of conical bodies having the same constitution 

 as those just described, fig. 45. The perfect resemblance between 

 these and the prehensile organs of the Diphydje cannot be overlooked. 



66. The structure of the generative organs is still more instructive. 

 In the Medusffi I have endeavoured to show that there are always 

 processes of the two foundation membranes, the generative elements 

 being developed between them, figs. \ a, \\ a, xZ a, 2b a. 



(:>"]. In the Diphydse (and as I have good reason for believing in 

 the Physophoridae also) the generative organ commences as a simple 

 process of the common tube (fig. 39 a), and undergoing great changes 

 of form in the course of its development {b, c), it becomes at last 

 exactly similar to an ordinary natatorial organ with a sac composed 

 of two membranes suspended from its centre, fig. 39. In external 

 form it greatly resembles such a Medusa as Cytais, and this resem- 

 blance is much heightened when, as in some cases, it becomes detached 

 and swims freely about, fig. 41. The ova or spermatozoa, as the case 

 may be, are developed between the two membranes of the sac, the 

 inner of which at any rate is a continuation of the inner membrane of 

 the common tube, fig. 39. 



