TYPE PLATyHMLMINTHES. 



137 



The nervous system consists of a bilobed brain lying in 

 the anterior part of the body and 

 from wliich two nerve-cords pass 

 backwards, united at intervals by 

 cross-commissures and giving off 

 on their outer sides branches 

 which anastomose with one an- 

 other, forming a network. 



In Qunda seginentata the transverse 

 commissures agree in number and ar- 

 rangement with the lateral branches on 

 the one hand and with the diverticula of 

 the intestine on the other, the arrange- 

 ment of the two systems producing an 

 appearance of metamerization which is 

 most striking, especially as it affects as 

 well the excretory and reproductive sys- 

 tems. In this form an indication is 

 afforded of the manner in which the 

 more pronounced and typical metameri- 

 zation of the higher types has been pro- 

 duced by the more or less completed g 

 multiplication of the organs and the 

 integration of the parts so formed into 

 a metamere (see p. 48). 



Byes are usually present, fre- 

 quently provided with lenses, and, 

 though usually two in number, 

 may be very numerous and situ- Pig. 73.— A Teiclad Turbbllari- 

 ated along the margin of the body. '^'^' Sy^<^«>l^^«^ pellucidi^m (after 

 No otocysts occur, and the sides 

 of the anterior end of the body 

 are in some forms produced into 

 more or less elongated processes 

 which may possibly be mainly 

 sensory in function, while behind 

 them are areas of strongly ciliated 

 cells richly supplied with nerves 

 and presumably corresponding 

 with the ciliated depressions oc- 

 curring in the same region in the Alloiocoela and Rhabdoccela. 



Wheeler). 



ex = excretory system. 

 g = accessory gland. 

 ggl = vaginal glands. 

 n = nervous system. 

 od = oviduct. 

 ov = ovary. 



p = reproductive orifice. 

 ph = pharynx. 

 t = testis. 

 u = uterus. 

 vd = vas deferens. 

 vi = yolk-gland. 



