116 



HYDROZOA. 



Cathariiiensls, it is surrounded by a series of little 

 sacs, each well packed with thread-cells. 



The size and shape of the nectocalyx in relation 

 to the polypite with which it is connected may also 

 vary considerably. The veil which surrounds the 

 open margin of the nectosac in no case appears to 



Fiff. 23. 



Morpholog-y of Medl'std^ : — a, Mednsid, seen in i^rofile ; b, 

 the same, yiewed from below ; c, its pol}-pite ; d, part of its mar- 

 ginal canal, and other structures in connection therewith ; — v, 

 nectocalyx; tt, polj-pite ; x'> rQ^irginal canal; ;//, yeil; t, tentacle; 

 X, radiating canal ; u, reproductiye organ ; 0', coloured spot ; 0", 

 marginal vesicle. 



he absent. More than four longitudinal canals 

 sometimes occur. In Willsia these canals are seen 

 to bifurcate, each branch again dividing into two 

 others, so that, in this form, the six canals open by 

 twenty-four ducts into the circular vessel, (Jig, 24, 

 c). In Cunina, ^gina, and jEginopsis, both cir- 



