196 TEXTBOOK OF ZOOLOGY 



The inner body wall of each segment is made up of an inner longi- 

 tudinal layer and an outer circular layer of muscle. Segmental nerves 

 which are derived from segmental nerve ganglia innervate the meta- 

 meres and coordinate the movements of the body. The segmental 

 ganglia communicate with each other through connections extending 

 from one segment to the other. At the anterior end is the brain, 

 which is composed of a suprapharyngeal and a subpharyngeal gan- 

 glion joined together by a pair of commissures. The brain, however, 

 has little to do with the coordination of different parts of interseg- 

 mental and intrasegmental reflexes, so that the stimulation in one 

 segment automatically stimulates the adjoining ones. Reactions which 

 require immediate coordination of the whole body are controlled 

 by three giant nerve fibers which run through the entire length 

 of the nerve chain. The primary function of the suprapharyngeal 

 and subpharyngeal ganglia is to relay sensory impulses. 



The principal vessels of the circulatory system are a dorsal one, 

 through which the blood moves forward, and a ventral one through 

 which the blood moves posteriorly. These are connected in the an- 

 terior region of the body by a varying number of paired, segmental 

 hearts or connectives. The dorsal vessel exhibits wavelike contrac- 

 tile movements (peristaltic contractions) which force the blood 

 anteriorly. The latter passes through the hearts, which also pulsate, 

 then backward through the ventral vessel to the skin, intestine, and 

 other organs. Hemoglobin is suspended in the blood plasma of 

 some Chaetopoda ; in others, a green pigment known as chlorocru- 

 orin is found ; in still others no known blood pigment occurs. The 

 principal vessels and hearts have valves on their inner surfaces 

 which prevent the blood from flowing in the wrong direction. 



The class Chaetopoda may be divided into two orders; namely, 

 (1) the Polychaeta and (2) the Oligochaeta. 



Order Polychaeta. — The polychaetes (majiy bristles) are typically 

 marine Chaetopoda. One of the most widely known forms of this 

 group is Nereis virens or the clamworm, which may be studied as a 

 representative form. It possesses many setae (chaetae) located in 

 fleshy parapodia. In this case the parapodia with their setae con- 

 stitute the segmental appendages. The parapodium is divided into 

 a dorsal notopodium and a ventral neuropodium, and each surrounds 

 a large seta, or aciculum, which serves as a point of attachment for 



