4 THE SUB-KINGDOM CCELENTEIIATA. 



oval in form, and containing a fluid in their 

 interior. Tiie outer wall of the sac is entire and 

 very delicate ; the inner one is much stronger, having 

 its open extremity produced into a stout, rather 

 fusiform, sheath, Vv^hich terminates in a long thread, 



Fig. r. 



TJrticating organs of Ccelenteeata: — a, e, and/, thread-eeils of 

 Caryo'phyllia Smithii ; h, tliread-cell of Corynactis Allmani ; (\ 

 portion of the marginal canal of Willsia stcllata, with peculiar 

 receptacle, containing thread-cells, arising therefrom ; d, a single 

 thread-cell of the same ; g, thread-cell of Actinia (or Bunodes) 

 crassicornis. {AM magnified.) 



or 'ecthorseum.' A number of barbs or hooks are 

 sometimes disposed spirally around the sheath, 

 the ecthoraeum itself being often delicately ser- 

 rated. In the ordinary condition of the thread- 



