HISTOLOGY OF HYDRA 



85 



(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 presence of muscular 

 fibers (myofibrils) in the basal parts of the cells. 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 



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

 tory granules. (From Schneider.) 



pseudopodia by means of which Hydra attaches itself to the 

 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 

 tentacles 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 



