312 THE EARTHWORM AND OTHER ANNULATA 



by undulations of its body and by the movement of its lateral 

 appendages, or parapodia; but it soon burrows again into the 

 bottom where it lies for the most part with only the head exposed. 

 It is characteristically a form adapted for movement and light 

 burrowing under stones, among seaweed, in clean, loose sand and 

 fine gravel, or very soft mud, rather than for burrowing in close, 

 hard material. At the breeding season, in the spring, the male 

 and female worms swim freely near the surface, discharging their 

 ova and spermatozoa into the open water where fertilization 

 occurs. Their breeding habits are thus in marked contrast with 

 those of the earthworm and other oligochsetes. There are many 

 other species of Nereis at the seashore in all parts of the world. 



The external features of Nereis (Figs. 151 and 152) are also 

 noticeably different from those of Lumbricus (c/. Fig. 137 A). The 

 body is worm-like, 30 cm. or more in length, tapering toward the 

 two ends, but Ijroader at the anterior end where there is a distinct 

 head. The living animal is flesh-colored with a greenish sheen and 

 some iridescence as it is seen shifting its position in the water. On 

 the head are well developed sense-organs in the manner of a free- 

 living animal. The peristomium, or most anterior segment, like 

 that of the earthworm, bears a prostomium dorsally above the 

 mouth, but this prostomium is terminated anteriorly by a pair of 

 tentacles. In addition, there are on either side of the head four 

 tentacle-like organs of touch, the cirri, and antero-ventral to these 

 on either side, two palps. The month is ventrally placed below 

 the prostomium, with wrinkled lips encircling its outline. In 

 seizing prey the mouth is everted as a short proboscis at the free 

 end of which is a pair of powerful jaws. On the dorsal surface 

 of the prostomium are two pairs of simple eyes or ocelli. By 

 means of these organs of the head and its powers of locomotion, 

 Nereis is well equipped for the active existence which it leads in 

 preying upon smaller organisms such as other worms and the 

 small crustaceans that abound in its normal habitat. 



The median portion of the body is composed of segments, or 

 metameres, hke those of the earthworm ; but each of these segments, 

 except the peristomium, bears a pair of lateral appendages, the 

 parapodia (Fig. 152 B), which are paddle-like organs used in locomo- 

 tion and respiration. Anteriorly, the parapodia are somewhat 

 reduced; and posteriorly, where the new segments are added as 

 the worm grows, they are somewhat undeveloped. There is, 



