HISTOLOGY OF HYDRA 



87 



formance 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 

 organism. 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 



