90 THE EVOLUTION OF MAN 



differentiated types the sense organs, the eye, the ear, the 

 organs of touch, of taste, and of smell, are concerned with 

 supplying the central apparatus with those elements out of 

 which the intellectual life is built. This function must be en- 

 tirely superfluous in such an animal as a sea-anemone where 

 no central organ exists. Here the sense organs are not con- 

 cerned with sensations; they merely excite muscles to action, 

 a function which they also exercise in the higher animals. 

 They are receptors for a multitude of external changes and 

 when thus excited they serve as triggers, so to speak, to set 

 off the subjacent muscles. Since they are not concerned with 

 sensations they are more appropriately designated as receptors 

 than as sense organs and hence the term receptor, which is the 

 more inclusive of the two, is the better one to employ. Sea- 

 anemones, therefore, represent a more primitive type of neuro- 

 muscular mechanism than the higher animals do, one in which 

 of the three organs, receptor, adjuster, and effector, only the 

 first and last are present, the adjuster, or central organ, being 

 a later acquisition. 



SPONGES 



If receptors and effectors were developed before adjusters, 

 it is natural to ask whether of these two parts one preceded the 

 other or did they both evolve simultaneously? Among the 

 multicellular animals lower than the sea-anemones, the sponges 

 throw light on this question. A single sponge is a goblet-shaped 

 or finger-shaped animal attached to the sea-bed. Its outer 

 surface is covered with pores which lead into a system of 

 canals provided with lash-cells by which the water is moved 

 through the canals to a large space in the middle of the sponge 

 from which this fluid escapes by a conspicuous opening at the 

 unattached end. From the current of water thus passing 

 through the sponge the animal extracts its nourishment and the 



