PHYLUM ANNELIDA 277 



while in Lumbricus the commissures are enlarged and muscular, func- 

 tioning as "hearts." Beneath the ventral vessel small longitudinal vessels 

 parallel the nerve cord, and carry blood posteriorly. 



In each body segment, the ventral vessel gives off paired branches 

 to the body wall and median unpaired branches to the intestine. The 

 dorsal vessel receives similar branches. Some of the blood passes close to 

 the intestinal gastrodermis where it picks up nutrients, some passes be- 

 neath the skin (and in Nereis through the parapodia) where it is 

 oxygenated, and a little passes to the nephridia, giving up wastes. All of 

 this blood is mixed together dorsally in each segment. 



In addition, a pair of commissures in each segment carry some blood 

 directly from ventral to dorsal vessels. In the middle region of the body 

 these are small and tortuous, forcing most of the blood through the 

 capillaries, but posteriorly they are more prominent, permitting a fairly 

 free flow of blood around and around the whole circulatory system. 



The blood contains dissolved hemoglobin which greatly facilitates 

 the transport of oxygen and carbon dioxide (p. 90). 



The major advance of the annelid system over that of the nemer- 

 teans is the addition of the capillary networks, a much more finely 

 branched system which is an efficient mechanism for distribution. 



1 24. Nereis and Lumbricus: Excretory System 



Each segment except the first and last contains a pair of metaneph- 

 ridia, convoluted tubules lying in a vascularized, glandular mass of 

 tissue. The mass lies at the base of each neuropodium in Nereis and 

 against the anterior sejJtum in Lumbricus. From each nephridium the 

 tubule extends forward through the septum to open as a ciliated funnel 

 in the coelom of the next anterior segment. The other end of the tubule 

 opens to the exterior at the minute excretory pore. 



The funnel collects coelomic fluid, including some debris from the 

 chloragen cells, and passes it down the tubule. Along the way the fluid 

 is modified so that only waste remains in the portion excreted. In Lum- 

 bricus a terminal expansion of the tubule forms a bladder. 



125. Nereis and Lumbricus: Reproduction 



The reproductive systems of polychaetes and oligochaetes are very 

 different. Gonads appear in Nereis only during the breeding season, 

 developing from the peritoneum lining the ventral body wall in many 

 of the segments. Eggs or sperm accumulate in the coelomic cavities and 

 are eventually shed through temporary ruptures of the body wall. Fer- 

 tilization is left to chance in the open sea water. 



In some species of Nereis, and in many other polychaetes, the gonads 

 appear in the posterior half of the body, which becomes considerably 

 modified as the gametes accumulate. The parapodia develop foliaceous 

 outgrowths and the chaetae become larger and often flattened. The eyes 

 may become temporarily enlarged. On the night of breeding the individ- 

 uals leave their burrows and swim to the surface, the enlarged parapodia 



