V PHYLUM PLATYHELMINTHES 295 



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. Statocysts 

 containing statoliths have been found in only a few of the 

 Nemerteans. 



Reproductive System. — Most species are diceeious. The ovaries 

 (Fig. 228), ov.) and testes are situated in the intervals between the 

 intestinal ca'ca. 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, rarely 

 to the ventral surface, on which it opens by a pore. In all 

 probability the cavities of these hollow gonads are all that 

 represent the coelome of higher forms. 



Development. — Some of the Nemerteans go through a meta- 

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

 teristic larval form is the pilidmm (Fig. 236). This is a helmet- 

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

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

 ectodermal invaginations, growing inwards around the intestine, 

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

 worm, which subsecjuently 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 segmcntata 

 (p. 255) among the Turbellaria, a serial repetition of the internal 

 parts (pseudo-metamerism, associated with the presence of 

 regularly arranged transverse partitions of dorso-ventral muscular 

 fibres). 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 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. 



Order 1. Protonemertini. 



Dimyarian Nemertines with the lateral nerve-cords situated 

 outside the muscular layers. The mouth is situated behind the 

 brain. The proboscis is devoid of stylet. 



