ANNULATA (rING-WORMS) 145 



The nervous system consists of a bilobed dorsal ganglion, or 

 brain, situated in the anterior part of the head, connected 

 around the mouth with the anterior end of the ventral nerve cord 

 which extends to the posterior end of the body. This cord is in 

 reality double, each part with a separate sweUing or ganglion in 

 each segment, but both united into one by a common covering. 



The sense organs include the tentacles and cirri as probable 

 organs of touch, the palps used in testing food, and the four 

 simple eyes. Each eye consists of (i) the outer cuticle, the 

 continuation of the body chitin, (2) the cornea, the flattened 

 cells of the epidermis, (3) the large gelatinous lens, (4) the retina, 

 composed of many radially arranged cells, one end of each of 

 which is continuous with the optic nerve and the other projects 

 towards the lens as a clear, glass-like rod. The body of each 

 retinal cell is densely pigmented for the absorption of excess 

 light rays. 



The sexes are separate. At certain times during the summer 

 they swim near the surface and cast the ova and spermatozoa 

 into the water, where union between them later occurs. The 

 fertilized egg develops into the adult through a series of radically 

 different forms, one of which, the trochosphere stage, it possesses 

 in common with mollusks as well as with other worms. 



Preservable as fossils are the chitinous jaws, trails, excrement 

 and burrows. 



1. Examine Nereis, noting the black jaws at the end of the 

 protrusible pharynx, the eyes, body segments, parapodia, bristles. 



2. What is the habitat of the animal ? 



3. Its food and how procured ? 



4. How is the food digested and the waste moved through 

 the digestive canal ? 



5. How is the digested food absorbed and carried throughout 

 the body ? 



6. Describe briefly (a) respiration; (b) excretion; (c) the 

 nervous system ; (d) sense organs. 



7. What are the principal muscles and their uses? 



8. What fossil record can Nereis leave of itself ? 



