EARTHWORM 343 



(chaetae), or bristlelike appendages on the body segments. The 

 setae are chitinous and are embedded in pits of the integument. 

 They bear muscle attachments which make them movable and there- 

 fore useful in locomotion. The coelom, which surrounds the straight 

 digestive tract, is divided between the segments by partitions known 

 as septae. Typically, each coelomic space possesses a pair of nephridial 

 tubules which communicate with the coelom at one end by means of 

 a ciliated, funnellike opening, the nephrostome. The other end opens 

 to the outside by means of a nephridiopore. The nephridia remove 

 nitrogenous waste materials from the coelomic cavities and from the 

 blood. 



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 supra-pharyngeal 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 sub- 

 pharyngeal 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 gr connectives. The dorsal ^vessel exhibits wavelike contrac- 

 tile movements (peristaltic contractions) which force the blood an- 

 teriorly. 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 chlorocruorin 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 opposite direction. 



