198 



ZOOLOGY. 



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

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

 the other behind ; these thickenings push inwards, and 



are the 

 future 



germs 

 worm. 



of the antei'ior 

 The anterior 



and posterior end of the 

 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 

 narrow slit remains. The 

 disks now divide into two 

 layers, the outer much 

 thicker than the inner. 

 Soon the anterior pair of 

 disks imite, and the head . 

 of the worm is soonformed, 

 when the elliptical outline 

 of the flat worm is indi- 

 cated, and appears some- 

 what as in Fig. 136. The 

 yolk mass, with the alimentary canal of the Pilidium, 

 is taken bodily into the interior of the Nemertes, the 

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

 bottom. 



The free-swimming larvse of other Nemerteans are very 

 closely similar to those of the Annelids, so 

 that from this fact and the nature of the 

 highly developed circulatory system, the 

 Nemerteans have been removed from the 

 neighborhood of the flat worms, and placed 

 near the Balanoglossus and Oephyrea, as 

 well as the leeches. 



Order 1. Ano2}la. — In this group the pro- 

 boscis is without a style. The species of 

 Linens and Meckelia are, in some cases, 

 very long. Meckelia ingens Leidy is 3j centimetres (an 

 inch) wide, and attains a length of 4 metres (loj feet). It 



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

 tee, with the worm growing in it. v, velum ; 

 e, eyes ; i, intestine of the Nemertean worm. — 

 After Leuckart. 



Fig. 137. — Tetra- 

 Bfrrama^ aNemertean 

 worm ; magnified. 



