LITERATTJBE OF THE SUB-KINGDOM CCELENTEEATA. 425 



similarity in outward form and habit had long kept concealed from 

 the notice of observers apparently by no means wanting in patience 

 or ingenuity. A peculiar feeling of pleasure, therefore, attends the 

 perusal of the works of those investigators, whose skilful use of such 

 opportunities as lay within their reach, first led them to form right 

 conjectures as to the proper affinities of the Siphonophora. 



Xike most CorynidaB and Sertularidse, these creatures are, in 

 general, of composite structure, presenting a hollow stem, or coeno- 

 sarc, by which numbers of digestive zooids (polypites) are connected. 

 G-enerative bodies, agreeing in all respects with those of the fixed 

 Hydrozoa, are also borne on the same stem. Thus the mode in which 

 both groups of organisms discharge the two great processes of nutri- 

 tion and reproduction, save as to a few comparatively unimportant 

 matters of detail, is precisely similar. It is otherwise with the func- 

 tions of relation. For in the Siphonophora, the entire compound mass, 

 not, as in the Corynidie and Sertularida?, permanently fixed by one of 

 its extremities to some fragment of rock, shell, sea-weed, or other 

 foreign support, is free and endowed with the power of executing a 

 variety of graceful movements, due, for the most part, to a very simple 

 arrangement of contractile tissues, and further assisted, in many cases, 

 by the addition of special organs which are at once seen to bear a 

 close resemblance to the swimming-bells (nectocalyces) of the Cryp- 

 tocarpse. But, while in the members of this group, a single digestive 

 zooid is, in a manner, suspended from the roof of its swimming-bell 

 and thus brought into immediate connection with the canal system of 

 the latter, — in the Siphonophora the polypites and nectocalyces are 

 attached independently to different parts of the common trunk, 

 through which alone a distant communication between their cavities 

 is maintained. Besides the reproductive organs, polypites, and swim- 

 ming bells, (these last being sometimes absent) many other append- 

 ages may arise from the coenosarc of the Siphonophora. In one large 

 section of the group its anterior extremity forms a float-like expan- 

 sion, or pneumatophore, within which a hollow air-vesicle, the pneu- 

 matocyst, is lodged. Such forms have now been placed in a separate 

 order, Physophoridae, as distinguished from another division of equi- 

 valent value, the Calycophoriche, in which there is no air-vesicle, 

 while swimming bells are constantly present. 



- 



Thus the dismemberment of the old classes, Acalephas and Polypi, 

 may now be regarded as complete. But with reference alike to the 

 precise limits and nomenclature of those primary groups of Ccelente- 



zation of the Siphonophora presented. Another interval of two years elapsed, when 

 the chief difficulties of the subject were finally set at rest by the nearly simultaneous 

 publication of three memoirs on the part of Yogt, Kolliker and Leuckart, respec- 

 tively, wherein the structural features of several genera of Siphonophora were 

 reviewed in detail and illustrated by numerous figures. Both Leuckart and Gegen- 

 baur, in the course of the following year (1854), made further valuable contributions 

 to the same department of zoophytology. (For a full citation of all the works and 

 papers Iiere referred to, vid. the bibliography given by Huxley, op. s. cit.) 



