PHYLUM ANNl'LATA 



459 



'prost 



lies below the alimentary canal, the sub-neural below this again 

 under the nerve-cord ; the lateral neural lie on either side of the 

 nerve-cord. The tinnsvcrnc Irunclies correspond in number to the 

 segments ; they run round from the dorsal vessel to the ventral, 

 giving off branches in their course. Five of them, viz. those in 

 the seventh to the eleventh segments inclusively, are dilated and 

 pulsate rhythmically ; these have the function of driving the 

 blood through the system of vessels, and are hence frequently 

 termed the '' hearts." The walls of the principal vessels are 

 contractile, and assist in bringing about the movement of the 

 blood, which is propelled in such a way as to run forwards in the 

 dorsal vessel and backwards in the ventral, its direction of move- 

 ment being regulated by a number of valves in the " hearts," the 

 dorsal vessel, and the chief vessels connected with it. 



The nervous system (Fig. 361) consists of a dorsal biJobed 

 train or cerebral ganglion and a double ventral nerve-cord 

 together with a pair of mbo- 

 phageal connectives, by which 

 the former is connected with 

 the anterior end of the latter. 

 The brain, which is of small 

 size, is situated in the third 

 segment, above the beginning 

 of the alimentary canal ; it is 

 divided by a median constric- 

 tion into two lateral parts of 

 pyriform shape with their 

 broad ends in contact. The 

 connectives pass from this 

 round the sides of the ali- 

 mentaiy canal to unite in the 

 middle below with the anterior 

 end of the ventral nerve-cord. 

 In this way a complete nerve- 

 ring or nerve-collar surrounds 

 the anterior part of the enteric 

 canal in the third segment. 

 From this the ventral nerve- 

 cord extends backwards to the posterior end of the body, and in 

 each segment it presents a slight enlargement or ganglion, as it is 

 usually termed, most conspicuous in the more posterior segments. 

 The wiaole cord is double, consisting of two intimately united 

 right and left parts. From the brain, nerves are given off to 

 the prostomium ; and from the ventral cord three pairs of nerves 

 arise in each segment. From the cesophageal connectives a 

 series of stomatogastric nerves pass to the pharynx and neighbouring 

 parts of the alimentary canal. 



K^an^ 



Fli;. 3Lil,-— Iiumbricus. Anterior portion of 

 nervous system, cer. f/ang. cerebral ganglion 

 or brain ; com. cesophageal connectives ; ne. co. 

 ventral nerve-cord ; prost. prostomium. (After 

 Leuckart.) 



