376 



HISTOLOGY 



cells in the higher forms, however, and passed from cell to cell until 

 delivered for use at the surface. It is probably an organ of defense, and 



is usually surrounded by a layer 

 of slime that may contain poison. 

 The rhabdites are developed in 

 some cases into large structures 

 called stylets and, by some, these 

 are thought to be a transition 

 from the rhabdite into the next 

 form of offensive mechanical pro- 

 tection that we shall study, the 

 nematocyst or nettle cell of the 

 coelenterates. 



The nettle cell or cmdoUast is 

 an organ that is developed in the 

 cytoplasm of a single cell, and is a 

 marked example of a regular and 

 complicated structure of great 

 efficiency and delicate adjustment 

 formed by an apparently amor- 

 phous mass of protoplasm. It 

 much resembles the trichocyst of 

 some Infusoria with which it is 

 probably homologous (see Fig. 



245)- 



The cnidoblast or nettle cell is 

 found in the basal layer of the 

 epidermis in Hydra, and when 



FIG. 343- A, young but fully formed cnidoblast young it bears no trace of its 



future development and func _ 



tion. When called upon to de- 

 velop nematocysts, or stinging 

 sacs, by the need of them on the surface, it produces them in its 

 distal cytoplasm which is enlarged and drawn somewhat toward the 

 surface at this time. 



The nematocyst first appears as a small, rounded mass of greater 

 density than the surrounding cytoplasm and, as it enlarges, it develops 

 a less dense interior. Its distal wall is invaginated into this interior as 

 a hollow thread which, when developed, lies coiled in the space. The 

 external opening of the lumen of this invaginated tube thread is closed by 

 a thin cover. The interior of the sac-like portion or capsule is filled with 

 the secretion which is a fluid in the ripe organ. 



The mature capsule (Fig. 343) with its contained parts does not lie 



of Hydra; B, freshly discharged cnidoblast of 

 same animal; t., thread; nem., nematocyst; 

 CMC., cnidocil; nu., nucleus of cnidoblast. 

 (After SCHNEIDER.) 



