422 



ZOOLOGY 



SECT. 



-Jb 'rest 



vessel (Fig. 330, dors, v.) runs along the middle of the dorsal surface 

 between the body-wall and the intestine ; it is readily visible shining 

 through the former in the living worm. The ventral vessel (vent, v.) 

 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 transverse branches correspond in number to the 

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

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

 the sixth 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 all 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. 



The nervous system (Fig. 332) consists of a dorsal bilobed 

 brain or cerebral ganglion and a double ventral chain of ganglia, 



together with a pair of cesoplia- 

 geal 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 constriction into 

 two lateral parts of pyriform 



J. J. / 



shape with their broad ends in 

 contact. The connectives pass 

 from this round the sides of the 

 alimentary canal to unite in the 

 middle below with the most an- 

 terior of the ventral chain of 

 nerve-ganglia. In this way a 



FIG. 332.-Lumbricus. Anterior portion of complete nerve-ring or nerve- 

 collar surrounds the anterior 

 part of the enteric canal in the 

 third segment. From this the 

 ventral chain of ganglia extends 



backwards to the posterior end of the body. In each segment it 

 presents a slight enlargement, most conspicuous in the more 

 posterior segments. The whole chain is double, the swelling in 

 each segment consisting of a pair of intimately fused ganglia. 

 From the brain nerves are given off to the prostomium ; and from 

 the ventral chain three pairs of nerves are given off in each seg- 

 ment. From the oesophageal connectives a series of stomatogastric 

 nerves pass to the pharynx and neighbouring parts of the 

 alimentary canal. 



com. 



nervous system, cer. gang, cerebral ganglion 

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

 ventral nerve-cord ;prost. prostomium. (After 

 Leuckart.) 



