290 



ZOOLOGY 



SECT. 



Development. Some of the Nemerteans go through a meta- 

 morphosis ; in the others the development is direct. The 



characteristic larval 

 form is the pili- 

 dium (Fig. 241). 

 This is a helmet- 

 shaped body with 

 side lobes like ear- 

 lappets, and a 

 bunch of cilia re- 

 presenting a spike. 

 In the metamor- 

 phosis two pairs of 

 ectodermal invagi- 

 nations, growing 

 inwards around the 

 intestine, fuse to - 

 gether and form 

 the integument and 

 body- wall of the 

 future worm, which 

 subsequently frees 

 itself from its in- 

 vestment and deve- 

 lops into the adult 

 form. In others 

 there is a ciliated 

 creeping larva 

 called the " larva 

 of Desor" in the 

 interior of which 

 the larval worm is 



FIG. 241. 1, Pilidium with advanced Nemertine worm ; /q^,!^ 



B, ripe embryo of Nemertes from interior of pilidium. developed 



an. amnion, or investment of the embryo ; i. intestine ; 



Ip. lateral pit ; n. nervous system ; ce. gullet ; pr. pro- 

 boscis ; st. stomach. (From Balfour, after Butschli.) 



: n |"U p PA.QP r>f thp 

 pilidium. 



DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Nemertinea are ciliated, unsegmented worms with elongated 

 body, without distinct ccelome. There is an eversible proboscis 

 enclosed in a sheath and capable of being protruded to a great 

 length through an aperture situated usually in front of and 

 above the mouth. The intestine usually has distinct lateral 

 diverticula, and there is a posteriorly situated anus. There is a 

 blood- vascular system, and also a system of excretory vessels with 

 ciliary flames. 



