WORMS. 15 



bw represents the body-wall. On either side a paddle 

 {/>) may be seen, which is divided into a dorsal and ven- 

 tral portion (dp and vp) : s, s are the hunches of bristle- 

 like setae; r is the intestine, which runs the whole length 

 of the body; pc the space surrounding the internal organs 

 or viscera, and, therefore, known as the perivisceral cavity ; 

 its is a nerve ganglion, which is only one of a chain of 

 ganglia, or nerve centres. These extend along the ventral 

 surface of the body, connected by a nerve band consisting 

 of two chords, which unite to form the ganglia. There is 

 a ganglion to each ring, and it supplies nerves to its own 

 segment; jrg'are segmental organs, so called because they 

 are found in nearly every segment of the body. Some of 

 these organs act as kidneys, and others as oviducts. 



The two principal blood-vessels of the Nereis run the 

 whole length of the body: one (w) along the ventral sur- 

 face, and the other (dv) along the middle of the back, 

 which may be seen in a living specimen. The ventral 

 vessel gives off in every segment of the body, excepting a 

 few in the region of the head, two smaller vessels, one on 

 either side. Each of these divide into two branches, one 

 of which (vb) goes to the lower portion of the paddle, 

 while the other (//;) passes upward round the intestine. 

 The branch (bv) connects with the branch (db) in the 

 upper portion of the paddle, and this connection is prob- 

 ably made by the capillary vessels like CO. The blood, 

 forced backward from the head into the ventral vessel, 

 (lows into the branches (vb) and (ib). The branch (//>) 

 receives the blood from the vessel {db), and then flows 

 into the dorsal vessel (<-/</), where it may be seen passing 

 in waves towards the head. It is in the' numerous blood- 

 vessels of the paddles that the blood is purified by throw- 

 ing off carbon dioxide and absorbing oxygen from the air 

 in the water. In the living worm the red blood in the 

 minute capillaries may be seen through the delicate, trans- 

 parent walls of the paddles. 



