218 



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 germs of the anterior 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 



e , , \ 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 unite, and the head 

 of the worm is soonformed, 

 when the elliptical outline 



Fig. 142. Larva or "Pilidium" of Nemer- n f +1 1P fl n 4- WO rm is indi- 

 tes, with the worm growing in it. v, velum ; 01 



Af%rLeu'ckart stineoftheNemerteailworm '"~ cate d> an ^ appears some- 

 what as in Fig. 142. 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 Gephyrea, as 

 well as the leeches. 



Order 1. Aiwpla. In this group the pro- 



b SCiS iS without a St y le ' Tlie S P ecieS f 



Linens and MecTcelia are, in some cases, 

 very long. Meckelia ing ens Leidy is 2^ centimetres . (an 

 inch) wide, and attains a length of 4 metres (15^ feet). It 



