HISTOLOGY OF HYDRA 



83 



(Fig. 35). On the tentacles they are much flattened while in 

 other regions of the body their size is intermediate. The trunk 

 epithelial cells are more or less vacuolated. All forms of these 

 epithelial cells are characterized by the prolongation of the basal 

 parts of the cells into muscular fibers. These fibers run up and 

 down the body thus forming a complete longitudinal muscular 

 investment for the entire animal giving it, with the transverse 

 muscle processes of the endoderm cells, the power of movement 

 in all directions. 



In addition to the contractile power the.epithelio-muscle cells 

 of the pedal disc and of the tentacles have the power of forming 

 pseudopodia by means of which Hydra attaches itself to the 



FIG. 35. Epithelio-muscle cells from Hydra fusca showing myofibrils and secre- 

 tory granules. (From Schneider.) 



sub-stratum alternately .by tentacles and pedal disc and thus 

 moves from place to place. 



2. Nettle or Stinging Cells. These peculiar cells are typical of 

 coelenterates and are not found elsewhere although we have seen 

 analogous structures in the trichocysts of Infusoria. They are 

 absent on the pedal disc but are particularly abundant on the ten- 

 tacles and on the hypostome where they are arranged in groups, 

 usually one large one surrounded by a crown of smaller ones. 

 They are called nettle or stinging cells because of the presence 

 of a coiled thread which is thrown out when the cell is irritated. 

 The tip of the thread contains a trace of poison so that minute 

 animals struck by them are paralyzed and become an easy prey 

 for the tentacles and mouth. These cells, which are sometimes 



