184 



FOUNDATIONS OF BIOLOGY 



The structural elements of the nervous system of all 

 animals consist of cells known as nerve cells, or NEURONS. 



FIG. 99. Diagram of a 

 simple type of receptor-ef- 

 fector system, found in some 

 Hydra-like animals. It com- 

 prises receptors (6), or sense 

 cells, reaching to the body 

 surface (a), with basal nerve 

 net (c) connecting with mus- 

 cle cells (d). (Slightly modi- 

 fied, after G. H. Parker.) 



FIG. 100. Diagram of a 

 more complex type of recep- 

 tor-effector system, found in 

 some Hydra-like animals. It 

 comprises, in addition to the 

 receptor (6) with nerve net 



(c) and the muscle cells (e), 

 another nerve (ganglion) cell 



(d) interpolated in the nerve 

 net. a, body surface. (After 

 G. H. Parker.) 



In the lower forms these cells are permanently united so that 

 they form NERVE NETS which surround and permeate the 

 tissues which they stimulate to action. In more highly 



FIG. 101. Diagram of a primary sensory (sf) and motor (mf) neuron of the ventral 

 nerve cord of an Earthworm, showing their connections with the skin (ep) and the 

 muscles (Im) to form a simple reflex arc. cm, circular muscles; ep, epidermis; Im, 

 longitudinal muscles; me, motor neuron cell body (in a ventral ganglion), with fiber 

 (mf); sc, sensory neuron cell body with fiber (sf) entering ganglion to form synapses 

 with processes of motor neuron. See Fig. 68. (After G. H. Parker, and Rctzius.) 



