TRANSACTIONS OF THE SECTIONS. 79 



mation at the attached extremity of the polype chain, and in both polygastric and 

 monogastric fornif!, the same gemmation is continually going on among the prehensile 

 and reproductive organs. The gemmae, whether they are eventually to become 

 polypes, prehensile organs, or reproductive organs, are invariably at first simple, 

 double-walled processes, containing a cavity continuous with that of the common 

 stem of the animal, which is itself a double-walled tube. The Diphydse, whether 

 polygastric or monogastric, are invariably dioecious. 



The genus Rosacea, among the Polygastric Diphydae, is remarkable in possessing 

 only the anterior piece, which is gelatinous and hemispherical, like the umbrel of a 

 Medusa. If a peculiar dilatation — the float — were formed at the extremity of the 

 polype-chain of a Diphyes, we should have one of the Physophoridse. 



The genera. Rhizopkysa, Physalia, Athoryhia, Physophora, Stephanomia, Agalma, 

 Porpiia, and Velella, were described and their structure illustrated by diagrams, 

 without which the details would be unintelligible. Suffice it to say, that their forms, 

 however varied, are shown to be simple modifications of a common type, in the main 

 identical with that of the Diphydae. Thus, such a polype-chain as that of Rosacea, 

 if it developed a float, would be a Rhizopkysa. The Physalia is a Rhizopkysa with 

 its float disproportionately enlarged ; the Physophora, a Rhizopkysa which has de- 

 veloped lateral natatorial organs like those of a Diphyes. Again, the Velella may 

 be considered as a Physalia flattened out and having its air-sac divided and subdivided 

 by partitions, until it becomes a firm, resisting, internal shell. 



The same continual multiplication of parts by gemmation goes on among the Phy- 

 sophoridae as among the Diphydae ; and the structure and mode of development of 

 the young organs is essentially the same. Great variety is presented by the repro- 

 ductive organs, from the form of mere sacs to that of free swimming bodies, precisely 

 resembling Medusas, and developing the generative elements only subsequently to 

 their liberation. In Physalia, the female organs are free-swimming medusiform bo- 

 dies, while the male organs are simple pyriform sacs, which remain attached and 

 develope their spermatozoa ?"« situ. In the language of the " alternation theory," the 

 Physalia itself and the medusiform body would be two generations, and we should 

 be presented with the unexampled peculiarity of a male giving birth to a female. 



As a general conclusion, it may be stated that the Diphydae and Physophoridae are 

 essentially composed of two membranes, an outer and an inner, which the author 

 calls " foundation-membranes," since every organ is formed by the modelling into 

 shape of one or other, or both of these, commencing as a simple process or diverti- 

 culum, and assuming its perfect form by a gradual differentiation. The stomach has 

 no wails distinct from those of the general parietes. The reproductive organs are 

 always developed externally, and the thread-cell is found in all in the greatest abun- 

 dance. The author lays particular stress on the bearing of the latter fact upon clas- 

 sification, and shows that the same organ is met with in equal abundance only in the 

 Hydroid and Sertularian Polypes, the Medusids, Beroidee, and .'V.nthozoic Polypes. 

 A similar organ has indeed been also found in an Echinoderm, in certain Trematoda, 

 and perhaps, although the author is inclined to think that its presence in this case is 

 accidental, in Eolis ; but in none does it assume such a prominent place as in the 

 families mentioned. 



The author endeavours to show that this fact, combined with the radiate polype 

 form, and the composition of the body of two distinct membranes, forms a very good 

 positive character for a group embracing the Hydroid and Anthozoic Polypes, and 

 the Acalephae ; a group equal in importance to any one of the primary subdivisions of 

 the animal kingdom. The name of Nematophora, "thread-bearers," is proposed for 

 this group, in allusion to the characteristic diffusion of the " thread-c.eW." But this 

 group must be subdivided into two equivalent sub-classes. In the Hydroid Polypes, 

 the Diphydas, Physophoridae, and Medusidae, the stomach is not distinct from the 

 common parietes, and the reproductive organs are external. In the Anthozoic 

 Polypes and Beroidae, the stomach is distinct from the common parietes, and the re- 

 productive organs are internal. Some years ago Mr. W. S. MacLeay, when consulted 

 by the author, suggested the name of CEcioa (those which have their eggs under 

 cover, " housed") for the latter division, and that of Anoecioa for the former. Now 

 a mutual representation runs through these two groups. For instance, the Actinidse 

 represent the Hydra and its allies ; the Zoanthidae represent the Corynidae ; the Phy- 

 sophoridas seem to represent the Pennatulidae ; and the Medusidae, the Beroidae. 



