ANATOMV AND PIIYSIOLOGr OF ANNELIDS 167 



develop. There is abundant food for them here, but the 

 danger is great, for the surface is constantly being swept by 

 fishes that live on the pelagic fauna. 



The musculature of annelids has been treated of in the intro- 

 duction to this chapter. The nervous system is of the same 

 type as that we have studied in the lobster — a dorsal brain 

 and a double ventral nerve chain connected by a nervous ring 

 around the pharynx. From the ventral nerve-cord in each 

 segment branches go off to the two lobes of the paddles. 



The sense-organs of annelids vary greatly in the different 

 orders. In the earthworm they are few and simple. The 

 whole surface of the body bears sensory hairs, and as these 

 are the only sense-organs known in the earthworm thej^ 

 are probably (in different regions of the body) capable 

 of being irritated by mechanical, chemical, thermal, or 

 luminous stimuli. Nereis, on the other hand, has special 

 sense-organs. Four eyes rest on the brain. Each is a cup- 

 like depression of the skin. At the bottom of each are the 

 special sensitive cells connected with a prominent nerve going 

 to the brain. The tentacles of Nereis are doubtless tactile, and 

 the palps probably have also a taste sense. Thus we see that 

 a carnivorous form like Nereis has well-developed sense-organs, 

 enabling it to find its prey; while herbivorous species, like 

 the earthworm, lack such special sense-organs. Each species 

 has organs agreeing with its needs. 



