INVERTP]BRATE ZOOLOGY. 89 



Commissural vessels. The lateral steins communicate by vessels 

 which pass around the body. In the genital region, five pairs 

 of pulsating commissures, the " hearts," pass from the dor- 

 sal to the ventral vessel. In the intestinal region, commis- 

 sures unite the dorsal with the sub-neural. Other communi- 

 cations are through anastomoses of capillary terminations. 



Lateral supply vessels. The body-wall, intestines and other 

 organs are supplied by vessels passing from the main trunks. 

 These form capillary net-works about the parts supplied. 



4. Excretory System. 



Every segment except the first three possesses a pair of lateral Ne- 

 phridia (segmental organs). They are the largest in the segments 

 just posterior to the reproductive organs, and are best studied there. 

 Each nephridium is a convoluted tube, having a free end, which 

 floats in the coelomic cavity. This has a ciliated funnel-shaped 

 opening and collects excretory material from the coelom. The tube 

 passes through the dissepiment and empties externally in the suc- 

 ceeding segment. Each tube may be divided into a transparent, a 

 glandular, and a terminal portion. 



5. Nervous System. 



This consists of a brain or supra-oeHophageal ganglion, lying upon the 

 pharynx, and a ventral cord possessing ganglionic enhtrgentents in 

 each segment. The brain sends out anterior nerves to the prosto- 

 mium and unites with the ventral chain bj' a pair of lateral com- 

 missures. These send out nerves to the pharyngeal region. From 

 the cord pass out lateral nerves, a gcniglionic pair from each gang- 

 lion, and an i iifcrga iiglion ic jxiir fixmi the intervals between them. 



64. Study of a cross-.sectlon. Momit a microtome section cut from a 

 piece prepared as in 61, and try to interpret the parts sho-wni by means of the 

 anatomical knowledge gained by the dissection. The parts especially brought 

 oixt will be the typhlosole, the longitudinal blood-vessels, the chloragogue 

 cells, and the nervous cord. The study of the body- walls and the muscular 

 layers is only to be accomplished by sections. If the plane of section passes 

 through the bristles (setae), examine and compare with 65. 



65. Study of 2M>"(ipodia of a Polychaete (Nereis). This is the common 

 clam-worm found at low tide in mud-flats. Notice in an entire specimen the 

 lateral expansions of each segment which bet^r the setae. These are the par- 



