THE HYDRA AS A SIMPLE METAZOAN 



267 



muscles of the ectoderm are active and coordinated as compared 

 with the sluggish and more diffuse contractions effected b}^ the 

 endodermal muscle fibers. 



As the nervous system of hydra is quite representative of the 

 conditions found in coelenterates and other simple metazoa, its 

 organization will be described in some detail. Extending through- 

 out the ectoderm are nerve cells or neurones, which are connected 

 with one another by their processes (Fig. 128). Although it is 

 impossible to be entirely certain, it appears that these connec- 

 tions represent continuity and not mere contact of neighboring 

 cell processes, as seems to be the case between the specialized nerve 

 cells of higher animals. These nerve cells of hydra lie in the basal 

 region throughout the ectoderm of body and tentacles and near the 



Fig. 129. — Schematic representation of the ectodermal nervous system, 

 including a cnidoblast shown as an independent effector. 



en, cnidoblapt; ect, ectoderm; m.f., muscle fibril of large ectoderm cell; n.c, nerve cell; 

 n-s.c, neuro-sensory cell; s.c, sensory cell; s.l., supporting lamella. 



muscle processes (Fig. 129). Since they appear edgewise when 

 the ectoderm is seen in profile, one does not appreciate their shape 

 and connections until they are seen in surface views. In addi- 

 tion to these nerve cells, there are in the ectoderm three slightly 

 different types of setisory cells, since they evidently receive the 

 external stimuli at their outer ends. One type is present on the 

 tentacles, one on the hypostome, and one on the foot (Fig. 130, 

 A, E, and K). In the body there are nerve cells having sensory 

 processes extending to the surface, and hence constituting a 

 fourth type called neurosensory cells (cf. Fig. 129). It is probable 

 that both the sensory cells and the neuro-sensory cells have some 

 direct connections with the muscle processes; but in the main 

 these sensory cells connect with the nerve cells, which in turn 



