PHYLUM ANNELIDA 199 



passes posteriorly through the ventral one. Its movement is effected 

 by wavelike contractions in the walls of the dorsal vessel. It reaches 

 the parapodia and digestive tract through lateral branches of the 

 ventral vessel and is then returned to the dorsal one by parietal 

 branches. 



Each segment of the body except the peristomium has two nephridia 

 opening directly from the coelom to the outside. The nephridium 

 consists of a ciliated funnel, nephrostome, and a coiled tubule which 

 ends in its external opening, the nephridiopore. The nephridia serve 

 to convey the excretory and reproductive products to the outside. The 

 sexes are separate and there are gonads in all the segments except 

 those in the anterior end of the body. The sex cells arise from the 

 walls of the coelom and when ripe pass to the outside, fertilization 

 taking place in the water. The fertilized egg develops into a trocho- 

 phore larva, which metamorphoses into the adult animal. 



In the central nervous system there are two suprapharyngeal gan- 

 glia dorsal to the pharynx. These are connected by means of com- 

 missures to the suhpharyngeal ganglion ventral to the pharynx. A 

 nerve chain, composed of segmental ganglia joined by intersegmental 

 connections, extends posteriorly on the ventral side of the body to 

 the anal segment. Lateral nerves from the ventral nerve chain in- 

 nervate the various organs of the worm. Two eyes receive nervous 

 connections from the brain and the animal is apparently able to 

 detect moving objects. 



Order Oligochaeta. — The best known example of the order Oligo- 

 chaeta is Lumhricus terrestris, the common earthworm, which is used 

 almost universally as a laboratory specimen. Lumbricus is not as 

 common in the Southwest as are other large forms of earthworms, 

 (Dipocardia) but is used here as an example because it is so well 

 known and because its features represent so well those common to 

 the entire order. 



EARTHWORM 



The body of Lumbricus terrestris varies from six to fourteen inches 

 in length and gives the appearance of a number of rings joined in a 

 linear arrangement. The rings are the body segments, or meta- 

 meres, and vary in number up to 175. In the adult the number of 

 segments from the anterior end to the posterior end of the clitellum 



