188 THE RISE OF ANIMAL LIFE 



from mouth to anus, is surrounded by a around in the dark. The centrahzation of 



sheet of tissue, the peritoneum, which forms the nervous system, initiated in planaria, is 



the walls of the coelom, a very important carried much further in Neanthes. Not only 



cavity introduced among the annelids for has the nervous tissue concentrated into 



the first time and referred to earlier. The two large masses, but also each segment has 



coelomic cavity in the annelids is decidedly a similar enlarged ganglion. With this or- 



different from the body cavity of the round- ganization the animal has a well-developed 



worms which had no lining whatever. The means of coordination, a far cry from the 



coelom functions in nutrition, since a large nerve net of hydra. 



portion of the food and waste products dif- If Neanthes is placed in a dish of sea 

 fuse into the coelomic fluid and through it water, it keeps up violent swimming move- 

 reach their ultimate destination. ments, but if a test tube is placed in the 



The circulatory system of Neanthes is far container the worm backs into it almost 

 superior to the one found in tlie nemertines. instantly and quiets down. This is presum- 

 Tiny capillaries now allow a rapid ex- ably due to the tactile stimulation from the 

 change of oxygen and carbon dioxide, as tube wall, reminiscent of the burrow. Move- 

 well as food products. Furthermore, circu- ments during the breeding period also indi- 

 lation is maintained by contraction of the cate a highly developed neuromotor mecha- 

 large blood vessels themselves, rather than nism. 



by the body as a whole. Here, then, is seen In the nights of late July and August, 



for the first time a contractile portion of the Neanthes gather in great numbers along the 



system which serves as a heart to keep the eastern coast of the United States for the 



blood circulating continuously. The blood, purpose of shedding their eggs and sperms, 



like that of the nemertines, contains hemo- By illuminating a small area of an oppro- 



o-lobin althoueh it is not confined in cells, priate ocean surface, thousands of worms 



but is free in the plasma. may be seen at this time churning the 



Neanthes receives its oxygen and elimi- water by swimming at a rapid, erratic rate, 

 nates its excess carbon dioxide through the and swirling in apparent frenzy. Suddenly 

 thin walls of the parapods. The frequent the females seem to split open along their 

 waving movement of these organs facili- sides, discharging their eggs into the sea 

 tates rapid gas exchange. Each segment has water like a white cloud. The males, which 

 a pair of small coiled tubules with an inter- are smaller than the females, shed their 

 nal opening, the nephrostome, which resem- sperm in a similar manner. Both sexes die 

 bles a tiny ciliated funnel, and an external shortly after extruding their gonadal prod- 

 opening, the nephridiopore, at the opposite ucts. By dipping up some of this water it 

 end, which perforates the body wall. The is possible to observe the early stages in 

 nephrostomes lie in the segment anterior to the embryology of this animal. The eggs 

 the tubule and the nephridiopores. They undergo segmentation and develop into 

 take in coelomic fluid from which nitrog- free-swimming ciliated larvae, called troch- 

 enous wastes are extracted in the tubule ophores, which promptly settle to the bottom 

 and excreted through the nephridiopore. and metamorphose into young Neanthes. 



This excretory system is considerably more 



1 ^1 X i- 1 • 1 Relatives of Neanthes 



complex than the one noted m planaria, 



although fundamentally it is similar. There are many close relatives of Nean- 



Neanthes shows more varied and specific thes living along the ocean shores, although 



responses to the external world than are some are found at great depths. They range 



found in lower forms. Its four eyes and from miscroscopic fomis to those that reach 



sensitive tentacles aid the worm in getting 10 feet in length. Some are active swimmers 



