94 



HYDRA. PHYLUM CCELENTERATA 



pierce the surface protoplasm. How these cells are connected is 

 not certain ; probably it is not by continuity of their processes 

 but, as in the synapses of higher animals (p. 512), by contiguity. 

 The function of a nervous system is to conduct impulses 

 started in it by sense organs, and thus to co-ordinate the move- 

 ments and reactions of the body. Fundamentally all nervous 

 systems work in the same way, but that of Hydra differs in some 





'^ 



'i:-.^ 





Fig. 61. — A small portion 

 of a transverse section 

 of Hydra X c. 500. 



ect.. Ectoderm ; end., endoderm ; 

 f.p., food particle, ingested 

 by an endoderm cell ; int.c, 

 interstitial cells ; tn.e.c, 

 musculo-epithelial cell ; m^c, 

 nematocyst ; st.L, structure- 

 less lamella ; vac, vacuoles in 

 endoderm cells ; vac'., vacu- 

 oles in ectoderm cells. 



Fig. 62. — A diagram of the 

 nervous system of Hydra. — 

 After Hadzi. 



respects from those of most other animals. The nerve cells form 

 a network, in which it is impossible to distinguish either nerves 

 or a central nervous system, such as vertebrates possess. Impulses 

 travel in the network equally well in all directions, and it is 

 therefore difficult to see how variation in behaviour is possible, 

 but there is in fact some. Most of our knowledge of the physiology 

 of the nerve net is based on that of the sea anemones (p. 116), 

 but there is no reason to think that Hydra is greatly different. A 

 stimulus, such as a touch, sets up in a sense cell a train or volley of 

 impulses, and the stronger the stimulus the greater their number. 

 Both the nerve-muscle junction, and to a lesser extent the synapse, 



