60 



ANIMAL FORMS 



form along almost any seashore. The body presents the 

 same segmented appearance as the earthworm, but the 

 head (Fig. 37, A) is provided with numerous sense organs, 

 chief among which are four eyes and 

 several tentacles or " feelers." 



The segments behind the head 





FIG. 37. A. head and one of the lateral appendages (B) of a marine worm (Nereis 

 brandtii) ; al. intestine ; /, " gill " ; k, kidney ; n, nerve cord ; s, bristles for loco- 

 motion. 



differ very little from one another, and, unlike those of 

 the earthworm, each bears a pair of lateral plates (Figs. 

 36, 37, B) or paddles with many lobes, some of which bear 

 numerous bristles. By a to-and-f ro movement these organs 

 aid in pushing the animal about, or may enable certain spe- 

 cies to swim with considerable rapidity. 



As in all other worms, respiration takes place through 

 the surface of the body, the area of which is increased by 

 the development, on certain portions of the paddles (para- 

 podia), of plates penetrated with numerous blood-vessels, 

 which thus become special respiratory organs or gills 

 (Fig. 37, B). 



In their internal organization the Polychaetes are con- 

 structed practically on the same plan as the earthworms, 

 the principal difference being in the reproductive system. 

 In the earthworm this is restricted to some of the forward 

 segments, while in the present group the eggs and sperms 



