TYPE ANNELIDA. 



239 



Oe 



in others the intestine bends upon itself and passes forward to 

 open on the dorsal surface near the anterior end of the body 

 (Fig. 109, A). Throughout the greater extent of the intestine 

 there runs along its ventral surface 

 a ciliated groove which is no doubt 

 homologous with the accessory in- 

 testine of certain Polychseta (see 

 p. 207). 



The nervous system partakes of 

 the absence of distinct metamerism 

 which characterizes the other parts. 

 It consists of a brain lying in the 

 anterior portion of the body above 

 the oesophagus and sending a com- 

 missure downwards and backwards 

 on each side to form the circuni- 

 cesophageal collar. These two com- 

 missures unite to form a single 

 nerve-cord (Fig. 109, ri) extending 

 the entire length of the body in the 

 ventral median line, differing from 

 the ventral cord of the Chsetopoda 

 in the absence of ganglia. Nerve- 

 cells are scattered along the entire 

 length of the cord and are not 

 aggregated into special ganglia, 

 though slight indications of such 

 an aggregation are found in Priapu- 

 lus. Nerves are given off at more 

 or less regular intervals on either 

 side, a somewhat metameric ap- 

 pearance being thus produced, but 

 the corresponding nerves of op- 



posite sides do not invariably arise ^ = veniral retractor muscle, 

 from the cord opposite each other. 



One, two or three pairs of nephridia (Fig. 109, ne) are as 

 a rule present and form conspicuous brown tubes, which com- 

 municate by a funnel with the body-cavity at one extremity 

 and with the exterior of the body at the other. They are 



FIG. 109. STRUCTURE OF Si- 

 punculus Oouldii (after AN- 

 DREWS). 

 A = anus. 

 Bs = blood vessel. 

 dR = dorsal retractor muscle. 

 Int = intestine. 

 N nerve- coid. 

 ne = nephridium. 

 Oe = O3soplmgus. 

 Ov = ovary. 



