ACTINOZOA. 151 



a thickened band, the 'strebla', twisted in a 

 screw-like manner around the basal portion of the 

 thread. The tangled cnidse are relatively broader 

 then the preceding, having a very long ecthorgeum, 

 loosely rolled up into a confused bundle. The 

 spiral cnidse present a much elongated, fusiform 

 chamber, within which the thread lies coiled in a 

 close regular spiral. Lastly, the so-called globate 

 cnidse have been seen to push out at each end a cy- 

 lindrical protuberance, sometimes equal in length 

 to the cnida itself, which does not contain any 

 thread. 



On the urticating organs of the Alcyonaria 

 less attention has been bestowed. In general, they 

 are of minute size and seem to resemble the 

 tangled cnidge of the Zoantharia, In Sarco- 

 dictyon they are aggregated on the tentacular 

 pinnae in minute rounded swellings, homologous 

 with the palpocils of the' fixed Hydrozoa. 



The thread-cells of the Ctenophoixc present a 

 peculiar structure. Each, in Pleurobrachia, ac- 

 cording to Professor H. J. Clark, appears of a 

 rounded or slightly napiform figure, and is covered 

 externally by a single, dense, layer of very minute 

 gi'anules. From the summit of a broad conical 

 projection on the inner surface of its otherwise 

 uniformly thick, but rather delicate, wall, arises, 

 in a very oblique direction, the simple thread, 

 which, after making not more than seven or eight, 

 equi-distant, spiral turns, set very far apart, ter- 

 minates suddenly in what seems to be a free 

 ending, precisely opposite its point of attachment. 

 The thread is cylindrical, smooth, apparently solid, 

 of firm consistence, and about eight or nine times 

 the length of its envelope, from which it is seli 



L 4 



