HISTOLOGY OF HYDRA 85 



of the single function of irritability. All cells of Hydra, like 

 all protoplasm are irritable and respond to external stimuli, but 

 the nerve cells are especially adapted in this respect, receiving 

 stimuli from the epithelio-muscle cells, from special sensory 

 cells or from other nerve cells and transmitting them to other 

 nerve cells and to the plexus of muscle fibers around the organ- 

 ism. While more numerous in the region about the mouth they 

 are not combined into special nerve centers or ganglia, but, like 

 the muscle processes, they form an interlacing network 

 throughout the animal. 



FIG. 37 Plexus of nerve cells in the ectoderm of Hydra fusca; the parallel 

 lines represent the longitudinal muscle fibers on the supporting lamella. (From 

 K. C. Schneider.) 



The small cell bodies are either bipolar or multipolar in 

 form, and fine fibers or processes extend from the poles often 

 for considerable distances into the surrounding tissues (Fig. 

 37). These fibers are the means of communication between 

 nerve cells, nerve cells and muscle processes and nerve cells 

 and nettle cells, and through their coordinating activities the 

 entire organism acts as a unit. 



4. Sensory Cells. Special cells for receiving external stimuli 

 are much more common in the endoderm than in the ectoderm, 

 but are found sparingly about the mouth and on the pedal 



