HYDROZOA. 



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least, is the life history of the Sertularidce. The 

 young Campanularia or Antennularia, at first 

 free, soon loses its cilia, fixes itself, and contracts 

 into a circular disc, which exhibits a division into 

 four lobes (fig. 10, c and d). In the centre of 

 the disc an opaque spot makes its appearance, and 

 over this the surface becomes gradually elevated, 

 until, finally, a young coenosarc is the result (fig. 

 10, e). From this, by gemmation, the branching 

 hydrosoma of the complete organism, with its 

 crowded assemblage of polypites, is subsequently 

 produced. 



Thus the young condition of a Sertularid would 

 appear to differ from that of a Corynid in having 

 a portion of its coenosarc more or less completely 

 developed before distinct traces of a polypite can 

 be observed. Such a conclusion accords well with 

 the composite structure always assumed by the 

 adult hydrosoma. And in this respect the Sertu- 

 laridce, while departing from the Corynidce, seem 

 to agree with the oceanic orders, Calycophoridce 

 and Physophoridce. 



Calycothouibje. Of the earlier embryonic 

 changes in the Calycophoridce little is known. 

 In Diphyes, according to Gegenbaur, the blasto- 

 derm at first appears as an elevated protuberance, 

 occupying only a portion of the segmented vitellus. 

 Soon, this blastoderm forms a rudimentary necto- 

 calyx, from which a short canal leads to the ciliated 

 cavity of the yolk below. The nectocalyx then 

 rapidly enlarges, while polypites are seen to arise 

 between it and the appended yolk-mass. 



Physophoribje. Our knowledge of the em- 

 bryology of the Physophoridce is confessedly scanty. 



