THE ORIGIN OF THE NERVOUS SYSTEM 



137 



THE ORIGIN OF THE NERVOUS SYSTEM AND ITS 

 APPROPRIATION OF EFFECTORS 



II. Receptor-effector Systems 



Br G. H. PARKER 



PROFESSOR OF ZOOLOGY, HARVARD UNIVERSITY 



THE second step in the development of the neuromuscular mech- 

 anism is represented by the receptor-effector system, a condition 

 fairly realized in such ccelenterates as the sea- anemones and the jelly- 

 fishes and probably recurring in the digestive tubes of the higher meta- 

 zoans. As an introductory example we may turn to the sea-anemones. 

 Most sea-anemones (Fig. 



1) are cylindrical animals 

 attached to some firm object 

 by their aboral disks and 

 carrying on their oral disks 

 a ring of tentacles surround- 

 ing the mouth. This aper- 

 ture leads inward through 

 a short gullet to a large, 

 somewhat divided, diges- 

 tive cavity, the gastro-vas- 

 cular space, which extends 

 throughout the whole in- 

 terior of the animal even 

 to the tips of its tentacles 

 and is the only cavity with- 

 in the sea-anemone. The 

 body of the animal is made 

 up of walls of extreme thin- 

 ness; these walls consist of 

 two layers of cells, an outer 

 one next the sea water, the ectoderm, and an inner one next the gastro- 

 vascular space, the entoderm. These two layers are separated by a 

 tough, non-cellular sheet, the supporting lamella. 



Unlike sponges, sea-anemones are very responsive to changes in 

 their environment. If a fully expanded Metridium is disturbed by 

 mechanical agitation, it will quickly retract its oral disk, discharge 

 through its mouth the water contained in its gastrovascular cavity, and 



vol. lxxv. — 10. 



Fig. 1. Longitudinal Section of a Sea- 

 anemone (Metridium) ; g, gullet ; gvs, gastro- 

 vascular space ; m, mouth ; t, tentacles. 



