316 



THE ANIMAL NERVOUS SYSTEM 



cell functions. Again, instead of speaking of the " nervous sys- 

 tem/' it is better to say the sensory-neuromuscular system, since 

 the receptors of the sense-organs, the adjustors of the nervous 

 system proper, and the effectors are intimately connected in a 

 unified system. Glands are also effectors but they are less in 

 evidence than muscles. The basis of function in this receptor- 

 ad justor-effedor mechanism is the irritability of the protoplasm. 

 Similar phenomena occur in the protozoa, and in some species 

 there is a miniature ''nervous" mechanism within the limits of 

 the cell (cf. Fig. 153 and Fig. 98, p. 183). The cell is the unit 

 in both protozoa* and metazoa. In the discussion that follows, the 

 organization of this sensory-neuromuscular system will be com- 

 pared in a series of animals with a view to tracing the probable 

 evolution of the nervous system and related structures. As the 

 behavior of animals is founded upon the nervous mechanism, we 

 may in conclusion refer briefly to the evolution cf nervous functions 

 and of intelligence. 



arc 



The Sensory-neuromuscular System 



Porifera. — In the sponges, or Phylum Porifera, which are the 



lowest type among the principal phyla 

 of many-celled animals {cf. Fig. 117, 

 p. 240), there is nothing that can be 

 called a nervous system and there are 

 not even nerve cells. The sponge 

 exhibits irritability, since it responds 

 locally when stimulated, and there is 

 a slow extension of the response in 

 all directions from the original place 

 of stimulation. Histological examina- 

 tion reveals that there are contractile 

 cells, particularly those arranged 

 around certain openings (Fig. 154). 

 These cells are independent effectors like 

 the cnidoblasts of hydra (cf. Fig. 129, 

 p. 267), since they receive and react to 

 stimuH independently of any other 

 part. The extension of the reaction 

 from an original region of stimulation is due to neuroid trans- 



_: w 



Fig. 154. — An apopyle, one of 

 the openings in a sponge, show- 

 ing contractile cells that func- 

 tion as independent effectors. 



apo, the apopyle; arc, archteo- 

 cytes or undifferentiated cells; my, 

 myocytes or contractile cells. 



