THE ORIGIN AND FUNCTION OF THE NERVOUS SYSTEM 



interesting story. According to this author contractile tissue develops before 

 any trace of the nervous system appears. In sponges, which are devoid of 

 nervous elements, the oscula open and close in response to appropriate stimuli. 

 These movements are brought about by a contractile tissue not unlike smooth 

 muscle. The active element or effector is thus the first to make its appearance, 

 and at this stage is brought into action by direct stimulation. Next in the order 

 of development is the sensory cell, derived from the epithelium in the neigh- 

 borhood of an effector, and specially differentiated to receive stimuli and trans- 

 mit them to the underlying muscle (Fig. 1, D). This stage of development is 

 reached by such ccelenterates as the sea-anemones. The advantage which 

 these forms derive from the specialized sensory cells or receptors is seen in the 

 character of their responses, which are more rapid than those of sponges. Such 



Cerebral ganglion- 



Esophageai connective --- 

 Pharynx 



Ventral nerve cord --^ 



.... Cerebral ganglion 

 -- Pharynx 

 Esophageai connective 



- Ventral nerve cord 



A B 



Fig. 2. Anterior portion of the nervous system of the earthworm: A, Lateral view; B, dorsal view. 



a sensory cell may be compared to a percussion cap through which a charge of 

 powder is ignited. 



But ccelenterates usually present a more complex arrangement of receptor 

 and effector elements than that indicated in Fig. 1, D. Fine branches from the 

 sensory cells anastomose with each other and form a nervous net within which 

 are scattered nerve-cells. Such a nerve net is seen in many parts of sea-ane- 

 mones (Fig. 1, E) and is well developed in the jellyfish (Fig. 1, F). It seems 

 capable of conveying nerve impulses coming from the sensory cells in all direc- 

 tions through the bell-shaped body of the jellyfish and to muscle-fibers far dis- 

 tant from the receptors involved. The conduction of nerve impulses from 

 receptors to effectors seems to occur diffusely through the net not in stated 

 directions nor along fixed paths. In this respect the diffuse nervous system of 

 the ccelenterates is in contrast with the more centralized system in the worms. 



