Chap. 28 ANNELIDS PIONEERS IN SEGMENTATION 567 



the inert soil. Clamworms and earthworms are illustrations of the saying that 

 the outside of an animal tells where it has been, the inside what it is. 



General Internal Structure. The internal structure of the clamworm is 

 essentially the same as that of the earthworm. Behind the esophagus the coelom 

 is divided into segments by partitions whose surfaces are covered with thin 

 peritoneum; there is a pair of kidneys in nearly every segment; and the nerve 

 chain is likewise branched. 



The jaws and the protrusible pharynx which can be thrust out onto the 

 prey are marks of the clamworm's predacious habit; withdrawing the pharynx 

 is a part of swallowing the food into the esophagus into which a pair of diges- 

 tive glands opens. From the esophagus, the digestive tube extends to the end 

 of the body. 



Reproduction. In Nereis and almost all polychaetes, the eggs and sperm 

 develop in separate individuals from certain cells in the peritoneum of most 

 of the segments. They are finally discharged into the water by way of the 

 nephridia. The breeding habits of these, like many marine invertebrates, fol- 

 low rhythms of the moon and the tides. In one of the smaller clamworms, 

 Nereis limbata, each breeding period follows a cycle of the moon. In each one 

 there are two peaks of abundance, also timed with phases of the moon. These 

 clamworms that throughout the year have lived on the sea bottom come to the 

 surface on certain days and hours, following a habit that probably began with 

 the great tides of the Cambrian Period, half a billion years ago. At Woods 

 Hole, Massachusetts, their swarming is a scheduled event of certain summer 

 nights. By a light held over the water, the throngs of swimmers can be seen 

 circling through the water as they shed eggs or sperm before they drop to the 

 bottom again. Each run begins near the time of the full moon, increases to a 

 maximum on successive nights, falls to a low point about the third quarter, 

 then increases again, and finally shortly after the new moon no worms appear. 

 The influences on the habits of these worms are examples of the many effects 

 that originate far away in space and time. 



Other Polychaetes 



Illustrations of a few polychaete worms may give a slight notion of their 

 variety and beauty. There is no hope of suggesting the translucence and play 

 of color of the living animals. Those that are mentioned here, or their near 

 relatives, live on both American sea coasts. 



The sea mouse. Aphrodite aculeata, may be three to seven inches long. The 

 under surface of the body is a flattened creeping sole like a snail's foot but is 

 furrowed by segmentation. Along each side of the upper part there is a band 

 of iridescent, hairlike setae. Between them the back is greenish gray bordered 

 by green and gold setae and brown spines that hide the segmentation. At first 

 glance, a sea mouse looks no more like a worm than it does like a mouse. 



