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 

 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 Coryiiidce and SertulaHdce the ec- 

 toderm excretes a firm, structureless, cuticular 

 lamina, to which the name of * polypary ' has been 

 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 (^Jig. 5, 6,). In such cases its 

 connection with the ectoderm is maintained by 

 means of transverse processes arising from the 

 latter, and these may present {fig. i(),h) consider- 

 able regularity of arrangement. The cup-shaped 

 chambers, or hydrothecae, commonly known as 

 polype-cells, which are so characteristic of the 

 order Seriidaridce, are merely prolongations of 

 this excreted layer. 



The polypites of the Ccdycophoridw and Physo- 

 phoridce are, in some genera, protected by over- 

 lapping appendages, termed bracts, or *hydro- 



D 2 



