v PHYLUM PLATYHELMINTHES 273 



Sometimes they are of extremely simple structure, consisting 

 merely of spots of pigment ; in other cases they are more highly 

 developed, having a spherical refractive body with a cellular 

 "vitreous body," and a "retina" consisting of a layer of mils 

 enclosed in a sheath of dark pigment, each rod having a separate 

 nerve-branch connected with it. Otocysts containing otoliths 

 have been found in only a few of the Nemerteans. 



Reproductive System. Most species are dioecious. The ovaries 

 (Fig. 214, en:) and tcstcs are situated in the intervals between the 

 intestinal cceca. The ovary or testis is a sac the cells lining which 

 give rise to ova or spermatozoa ; when these are mature each sac 

 opens by means of a narrow duct leading to the dorsal surface, 

 where it opens by a pore. 



Development. Some of the Nemerteans go through a meta- 

 morphosis ; in the others the development is direct. The charac- 

 teristic larval form is the Pilidiwm (Fig. 221.) This is a helmet- 

 shaped body with side lobes representing ear-lappets, and a bunch 

 of cilia representing a spike. In the metamorphosis two pairs of 

 ectodermal imaginations, growing inwards around the intestine, 

 fuse together and form the integument and body- wall of the future 

 worm, which subsequently frees itself from its investment and 

 develops 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 developed much as in the case of the 

 Pilidium. 



Though none of the Nemerteans exhibit metameric segmenta- 

 tion, yet in some of them there is, as in Gunda segmentata 

 (p. 241) among the Turbellaria, a serial repetition of the in- 

 ternal parts (pseudo-metamerism). Transverse fission is of 

 frequent occurrence. 



DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Nemertinea are ciliated, unsegrnented, worms with elongated 

 body, without distinct coelome. There is an eversible proboscis 

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

 length through an aperture situated 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. 



Sub- Class I Palseonemertinea. 



Nemertinea in which the head is devoid of deep lateral longi- 

 tudinal grooves, and in which the proboscis is not armed with a 

 stylet 



VOL. 1 'I' 



