212 ZOOLOGY. 



has been separated from the others. For the finer structure 

 of the body-walls see Fig. 146. 



The alimentary canal consists of a mouth, a pharynx 

 armed with two large teeth and much smaller ones. The 

 pharynx is entirely everted during the act of taking its food. 

 Into the (Esophagus empty two large salivary glands ; the 

 remainder of the alimentary canal is straight and tubular. 

 The circulatory system is very complicated ; it is closed and 

 the blood is red. Both the dorsal and ventral vessels ai-e 

 contractile, the blood flowing forward in the dorsal vessel, 

 and backward in the ventral vessel. The two small vessels, 

 one on each side, in each segment of the body, branch off 

 from the ventral vessel and subdivide, each sending a branch 

 to the ventral ramus of the foot of the segment behind, and 

 another larger branch around the intestine to the dorsal ves- 

 sel, receiving also, on its way, a vessel from the upper ramus 

 of the foot of its own segment. "Besides these principal 

 lateral vessels, there are five other vessels on each side 

 in each segment, coming from the ventral vessel. These 

 Jorm a loose but regular net-work tliat surrounds the in- 

 testine and is connected with five other convoluted vessels, 

 which join the dorsal vessel. This net-work on the intestine 

 probably supplies the hepatic organ ^ivith material for its 

 secretion, and very likely may receive nutritive material from 

 the digested food." (Turnbull.) 



The blood is aerated in the finer vessels of the oar-like feet 

 and in those situated about the alimentary canal. The 

 nen'ous system consists of the " brain"' and ventral double 

 ganglionuted cord. 



The sexes of Xereis virens are separate ; the eggs during 

 the breeding season fill the body-cavity, and pass out through 

 certain of the segmental organs, which act as oviducts, while 

 others, probably the more anterior ones, are excretory, like 

 the kidneys of vertebrates, as urea has been detected in them. 

 These organs are situated at the base of the lower ramus of 

 each foot. In some species of the CapitellidcB Eisig has found 

 that it is normal for several segmental organs to be present 

 in a single segment. 



While the mode of development of our Nereis has not 



