NATURE OF SENSE ORGANS 19 



So far as the neuromuscular system of the inverte- 

 brates is concerned, forms as low in the scale as the annelid 

 worms appear to possess all the elements of the corre- 

 sponding system in the vertebrates. Such worms may 

 have specialized receptors, eyes and the like, often of a 

 highly complex structure. They possess a well-differ- 

 entiated central nervous system as represented in their 

 so-called brain and ventral ganglionic chain. Finally, 

 they have an abundant variety of specialized effectors 

 in their various muscles, glands, and luminous organs. 

 Their receptors, central nervous organs, and muscles are 

 so related that reflexes can be demonstrated on them as 

 readily as on vertebrate preparations. In other words, 

 they possess in completeness, though in simple form, a 

 working neuromuscular mechanism essentially like that 

 of the higher animals. 



When, however, an examination of such forms as the 

 coelenterates is made, it is found that the coral animals, 

 the sea-anemones, the hydroids, and the like, possess 

 scarcely any trace of a central nervous apparatus. In 

 these animals fairly well specialized sensory surfaces 

 occur, whose nervous prolongations connect either imme- 

 diately with the subjacent musculature or give rise to a 

 nerve-net which in turn connects with the contractile ele- 

 ments. Thus the receptor is applied to the muscle very 

 directly and without the intervention of a central organ. 

 Such an arrangement allows of simple reflexes, for, when 

 the receptive surface is stimulated, the animal responds 

 at once by an appropriate muscular movement. Thus if 

 meat juice is discharged on the tentacles of a sea-anemone, 

 these organs carry out vermiculate movements and the 

 guUet opens; or if the pedal edge of the column is touched. 



