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PHYLUM PLATYHELMINTHES 273 



metimes 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 rods 

 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, ov.) and testes 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 Nernerteans go through a meta- 

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

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

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

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

 ectodermal invaginations, growing inwards around the intestine, 

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

 worm, Avhich 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, unsegmented, 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 



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