lOO ANATOMY OF THE HYDROSTATIC ACALEPH^ 



same. Great variety is presented by the reproductive organs, from 

 the form of mere sacs to that of free-swimming bodies, precisely re- 

 sembhng Medusae, and developing the generative elements only 

 subsequently to their liberation. In Physalia, the female organs are 

 free-swimming medusiform bodies, while the male organs are simple 

 pyriform sacs, which remain attached and develope their spermatozoa 

 in situ. In the language of the " alternation theory," the Pliysalia 

 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 Diphydse and 

 Physophorida; 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 walls 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 abundance. The author lays 

 particular stress on the bearing of the latter fact upon classification, 

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

 in the Hydroid and Sertularian Polypes, the Medusidse, Beroidae, and 

 Anthozoic 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 em- 

 bracing the Hydroid and Anthozoic Polpyes, 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-z&\\" But this group must be subdivided into two 

 equivalent sub-classes. In the Hydroid Polypes, the Diphydse, Phy- 

 sophoridse, and Medusidae, the stomach is not distinct from the 

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

 Anthozoic Polypes and Beroidse, the stomach is distinct from the 

 common parietes, and the reproductive organs are internal. Some 

 years ago Mr. W. S. MacLeay, when consulted by the author, sug- 

 gested the name of CEcioa (those which have their eggs under cover, 



