IRRITABILITY OF HYDRA 93 



cells. Contraction of the ectodermal cells results in shortening 

 of the body or in local movements of the tentacles. The 

 stimuli are received by either the sensory cells or by the ecto- 

 dermal epithelio-muscle cells, a function which has led to the 

 name neuro-muscle cells sometimes applied to them. The 

 impulse thus received is transmitted to the nerve cells especially 

 endowed with the power of transmission, and a general co- 

 ordinated reaction follows. 



The Neroous System. It is important to note that with the 

 nervous system a delicate function of protoplasm, irritability, 

 is singled out and made the special function of a complicated 

 series of cells and fibers. It is the centralizing and unifying 

 system of the organism, whereby the most widely separated 

 parts of the individual are made to act in harmony for the cap- 

 ture of food or escape from enemies. No such specialization 

 is found in protozoa apparently there is no need for it since 

 the single cell must act as a whole. In Hydra there is such need, 

 but we find that the nervous system or apparatus for co-ordinat- 

 ing muscular actions is extremely simple, consisting of a nerve- 

 cell plexus enveloping the whole animal. There is no evidence 

 of a centralized nervous system to which impulses due to ex- 

 ternal stimuli are sent and from which motor impulses to muscle 

 groups are transmitted. This comes first in higher forms, and is 

 accomplished by aggregation of nerve cells into ganglia of the 

 central nervous system. One part of this system, the brain, 

 with further advance and specialization, becomes the special 

 center for reception, analysis of external and internal stimuli and 

 for utilization and co-ordination of multifarious impressions and 

 responses, functions which we associate together under the 

 head of consciousness. The term loses its meaning w r hen used 

 to describe reactions of animals in which the co-ordinating 

 center has not reached a certain degree of complexity, but there 

 are undoubtedly different grades or degrees of consciousness, 

 just as there are different grades of complexity in the nervous 

 system. 



With Hydra there is no such centralization, but a step 

 in this direction is seen in Hydra and in the higher types of 



