156 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 wso- 
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 unite, and the head 
of the worm is soonformed, 
; when the elliptical outline 
ten Sita storm rows mk vlan of the flat worm is indi- 
SHioe Leuctact: — cated, and appears some- 
what as in Fig. 106. 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 larve 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. 
Fig. 107. — Tetra. Order 1. Anopla.—In this group the pro- 
ede ene boscis is without a style. The species of 
Lineus and Meckelia are, in some cases, 
very long. Meckelia ingens Leidy is 2} centimetres (an 
inch) wide, and attains a length of 4 metres (15} feet). It 
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