«m 





. 





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obse- 



i 



as 



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thee 



arge 



in ft 





HYDROZOA. 



35 



cellular, in others has its homogeneous periplast 

 traversed in all directions by a complex mesh- 

 work of threads, which remain quite distinct 

 from the endoplasts about which they diverge, and 

 with whose processes they appear never to become 

 continuous. The threads themselves seem elastic, 

 transparent, of different diameters, frequently 

 dividing, soon to unite again, and, occasionally, 

 disposing themselves side by side in such a manner 

 as to form extended plates. On the convex aspect 



hood, of the soft mass, which this thread system streng- 

 thens, the surface of the periplast is broken up 



into a number of polygonal cells, each furnished 



with an endoplast ; and in the delicate epithelial 



layer thus produced thread-cells may, without 

 difficulty, be observed. 



In the Corynidce and Sertularidce the ec- 

 toderm excretes a firm, structureless, cuticular 



x .' lamina, to which the name of ' polypary ' has been 

 V restricted by Professor Allman. This may so far 



separate itself from the outer surface of the ec- 

 ^ toderm as to present, at first sight, the aspect of 

 a distinct layer {fig. 5> b 9 ). In such cases its 

 connection with the ectoderm is maintained by 



means of transverse 



from 



{fig 



able regularity of arrangement. The cup-shaped 



chambers, or hydrothecse, commonly known as 



d wt.v polype-cells, which are so characteristic of the 



ndto order Sertularidce, are merely prolongations of 



la this excreted layer. 



ag 



7 .' 



but 



The polypites of the Calycophoridce and Physo- 

 phoridce are, in some genera, protected by over- 

 lapping appendages, termed bracts, or ^ hydro- 



r 



D 2 



