27 



of the rilidiuni, and grow tog'ether to surround the 

 endoderm. oi the hirva. When this process is complete 

 the bod}' wall of the animal, exclusive of the mesoderm, 

 now consists of four layers. Externalh' is the ciliated 

 ectoderm of the larva, and internally the endoderm, whilst 

 between these are two layers of ectoderm formed by the 

 fused invaginations. Of these two layers the inner 

 becomes the ectoderm of the adult worm, and corresponds 

 to the secondary ectoderm of Desor's larva. The young 

 ^"emertean continues to develop, and, when full grown, 

 parts company Avith the remains of the Pilidium, which 

 then consist of the original outer ectoderm and the outer 

 layer of the fused ectodermal invaginations. Hence the 

 discarded layer of primary ectoderm in Desor's larva 

 corresponds to the ectodermal shell of the Pilidium which 

 is cast off when the young A emertean escapes.* 



In several important respects the process of development 

 by Pilidium is said to diifer from that by the larva of 

 Desor. Among these may be more especially mentioned 

 the origin of the nervous system and of the nephridia. 

 The former is said to arise directly from the secondary 

 ectoderm as local thickenings of this layer, whilst in the 

 larva of Desor it has already been seen to take its origin 

 from the mesoderm. Again in Pilidium development 

 there is said to be an ectodermal cesophageal invagination 

 when the nephridia arise, and these are consequently not 

 of endodermal origin as in Desor's larva. 



There are also other differences, but the above are 

 sufficient to show that Hubrecht's account (which has 

 been followed above), though in its original form full and 



* An excellent account of Pilidium development, illustrated by numerous 

 coloured diagrams, is given by L. Joubin in Les Nemertiens, Traitc do 

 Zoologie de K. Blanchard, fascicule XI. Paris, 1897. 



