186 



THE RISE OF ANIMAL LIFE 



Fig. 10-9. The "sandworm" (Neanthes) is a common inhabitant of our Atlantic Coastal waters. It lives in a bur- 

 row in mud or sand from which its head and tentacles protrude. Small animals passing near enough are snatched 

 and devoured. During the breeding season worms leave their burrows and congregate in great numbers near 

 the surface of the sea. 



called parapods, which function like oars 

 on a boat to propel the animal through 

 the water. In addition, snake-like undula- 

 tions of the body aid in swimming. Paired 

 bunches of bristles (setae), located in the 

 parapodia, hold the animal in its burrow, 

 should an outside force attempt to remove 

 it. The head is a distinct structure well 

 provided with sense organs in the form of 

 four eyes, and two tentacles which appear 

 to be tactile in function (Fig. 10-10). 

 There is a protrusible pharynx which ter- 



minates in a pair of fierce-looking jaws. The 

 sturdy muscular body is covered with a 

 cuticle which takes on an iridescent sheen 

 in the sunlight. One is impressed by its 

 unique beauty as it glides through the 

 water with its graceful undulations. 



The tube-within-a-tube body plan is con- 

 spicuously evident when one studies Nean- 

 thes internally (Fig. 10-11). The internal 

 organs are serially repeated in each seg- 

 ment with the exception of the first and 

 the last. The gut, a straight tube passing 



pharynx extended 



pharynx retracted 



Fig. 10-10. Side views of the head of Neanthes showing the pharynx extended and refracted. 



