292 



ZOOLOGY 



SK( T. 



The excretory system has a considerable resemblance to that 

 of the Platyhelminthes. It consists of a pair of longitudinal 



vessels (Fig. 235, nepk.) which give off 

 branches, by one or several of which each 

 communicates with the exterior. The 

 fine terminal branches of the system are 

 provided with ciliary flames, each situated 

 in the midst of a group of cells, not in 

 the interior of a single flame-cell as in 

 the Flat-worms. 



There are no special organs of re- 

 spiration in any of the group. But 

 there is evidence that this function is 

 carried out, in part at least, by the taking 

 in and giving out of water through the 

 mouth by the oesophagus. 



The nervous system is in some re- 

 spects mor e highly developed than in the 

 Turbellaria. The brain (Figs. 228 and 



,-, - , T T-,. n^n \ i 



231, fr/\, and Fig. 230 cer. g.) is composed 

 of two pairs of ganglia, dorsal and ventral, 



the S an S lia of each pair .being con- 

 nected together by commissures, the 

 dorsal situated above, the ventral below, 



the anterior part of the proboscis and proboscis sheath, and 

 both being above the mouth and oesophagus. From the brain 

 pass backwards a pair of thick longitudinal nerve-cords which 



Fio. 231. Anterior portion of 

 the body of a Nemertine. 

 br. brain-lobes ; n. lateral 

 nerves ; p. o. external open- 

 ing through which the pro- 



FIGS. 232 and 233. Proboscis of a Hoplonemertean. with stylet reserve-sacs and muscular 

 bulb. Fig. 232 retracted, Fig. 233 everted. (After Hubrecht.) 



run throughout the length of the body. Usually these are 

 lateral in position, sometimes approximated dorsally, sometimes 

 ventrally. Usually the lateral nerve-cords meet posteriorly in a 

 commissure usually situated above, but in one genus below, the 



