198 



ZOOLOGY. 



side of the base of the velum (r) of the Pilidium ap- 

 pear two thickenings of the skin, one pair in front, 

 the other behind ; these thickenings push inwards, and 

 are the germs of the anteiior and posterior end of the- 

 future worm. The anterior pair become larger than 



the posterior ; the part of 

 the disk next to the oeso- 

 phagus thickens ; at the- 

 same time the alimentary 

 canal of the Pilidium 

 grows smaller, and only a 

 naiTOW slit remains. The 

 disks now divide into two- 

 layers, the outer much 

 thicker than the inner. 

 Soon the anterior pair of 

 disks unite, and the head 

 of the worm is soon formed,, 

 when the elliptical outline- 

 of the flat worm is indi- 

 cated, and appeal's some- 

 what as in Fig. 136. The- 

 yolk mass, with the alimentary canal of the Pilidium, 

 is taken bodily into the interior of the Xemertes, the- 

 Pilidium-skin falls off, and the worm finally seeks the- 

 bottom. 



The free-swimming larvae of other Xemerteans are very 

 closely similar to those of the Annelids, so 

 that fi-om this fact and the nature of the 

 highly developed circulatory system, the 

 Xemerteans have been removed from the 

 neighborhood of the flat woi-ms, and placed 

 near the BaJanogln^'iUS and Gepliyrea, as 

 well as the leeches. 



Order 1. Anojila. — In this group the pro- 

 boscis is without a style. The species of 

 Linens and Jleckdia are, in some cases, 

 very long. Jfeckelia ingens Leidy is 2^ centimetres (an 

 inch) wide, and attains a length of 4 metres (lo|r feet). It 



Fig. 136.— Larva or " Pilidimn" of Nemer- 

 tes, with the wonn growio^ in it. v, velum ; 

 e, eyes ; i, intestine of the J^mertean worm. — 

 After Leackart. 



Fig. 1.3T. — Telra- 

 gteinma^ aNemertean 

 worm ; magnified. 



