ANATOMY OF THE HYDROSTATIC ACALEPH^ 99 



which are terminated by small sacs, is developed from the base of 

 every polype. The small " prehensile " sac has a very peculiar form, 

 but is, morphologically, only a dilatation of its pedicle, one wall of 

 which is much thickened, and contains a great number of such urticat- 

 ing organs or " thread-cells " as are found among the Medusas. The 

 reproductive organs are medusiform bodies which are developed by 

 gemmation from the pedicle of the polype. 



The central sac of the medusiform body, instead of becoming a 

 stomach, developes the spermatozoa or ova within its walls. These 

 are generally shed forth while the organ is still attached, but in one 

 genus they swim about independently, and might readily be mistaken 

 for Medusae. 



In the Polygastric Diphydse new polypes are continually being 

 produced by gemmation at the attached extremity of the polype 

 chain, and in both polygastric and monogastric forms, the same gem- 

 mation 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 Diphydae, whether polygastric or monogastric, are invariably 

 dioecious. 



The genus Rosacea, among the Polygastric DiphydjE, is remark- 

 able 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 Physophoridae. 



The genera Rliizopliysa, Physalia, At/wry bia, Pliysophora, Ste- 

 phanomia, Agalma, Porpita, 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 Diphyds. Thus, such a polype-chain as 

 that of Rosacea, if it developed a float, would be a Rhisophysa. The 

 Physalia is a RliisopJiysa with its float disproportionately enlarged ; 

 the Pliysophora, a Rhizopliysa which has developed lateral natatorial 

 organs like those of a Diphyes. Again, the F^/(?//(3: may be considered 

 as a Physalia flattened out and having its air-sac divided and sub- 

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

 The same continual multiplication of parts by gemmation goes on 

 among the Physophoridse as among the Diphydae ; and the structure 

 and mode of development of the young organs is essentially the 



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