52 PHYLUM ANNELIDA 



movements of the body from one segment to another through 

 small openings in the septa. The red blood consists of a red 

 plasma, in which float colorless blood cells, and circulates in 

 closed tubes. The most important of these blood vessels are two 

 longitudinal tubes, the dorsal and the ventral arteries, which lie 

 in the median line, one above and the other below the alimentary 

 canal. The former, the dorsal artery, pulsates and drives the 

 blood toward the forward end of the body and distributes it to 

 lateral segmental arteries. Observe these and determine how 

 many there are in each segment ; also note the capillary network 

 into which the dorsal artery breaks up at its anterior end. The 

 dorsal portions of the lateral arteries carry the blood to the gills 

 and other organs, whence it collects again in the ventral portions 

 of these arteries and is conducted to the ventral artery. In this 

 vessel the blood flows toward the hinder end of the body. 



Exercise 6. Draw a view of the opened animal on a scale of 5, showing 

 the organs above described. Label all the organs carefully. 



Sever the ahmentary tract at the oesophagus and remove the 

 stomach intestine from the body. Observe the muscle bands in 

 the body waU ; note the difference in direction and size of the 

 different bands. Observe the muscles at the base of the acicula. 



The Excretory System. The kidneys of the animal consist of a 

 pair of very slender, tortuous tubes called nephridia, which lie 

 in the body cavity against the ventral body wall in each somite 

 except the last two or three; they are difficult to see because 

 each is covered with a mass of jellylike connective tissue. Each 

 nephridium opens through the body wall to the exterior in a 

 minute pore on the ventral surface of each somite near the base 

 of the parapodium. The anterior end of the nephridium passes 

 through the septum which forms the anterior wall of the somite 

 in which that organ lies, and opens into the body cavity. The 

 opening, which is ciliated, is called the nephrostome; it lies 

 against the anterior surface of a septum. 



The Nervous System. Observe in the midventral line of the 

 body cavity the nerve cord. Trace it forward to the brain. Note 

 the connectives which encircle the pharynx and connect it with 



