PLATYELMINTIIES. 167 



hollow imagination at the point where the two anterior discs fuse 

 in front. 



When the young Nemertine has become pretty well formed within 

 the Pilidium it becomes ciliated, begins to move, and eventually 

 frees itself and leads an independent existence, leaving its amnion in 

 the Pilidium which continues to live for some time. 



The central nervous system (fig. 94) is developed either before or 

 after the detachment of the young Nemertine, according to Metsch- 

 nikoff as a thickening of the epiblast. The young Nemertine is at 

 first without an anus. 



The development of the Nemertine within the Pilidium is clearly 

 identical with that of the Lineus embryo within the larval skin; 

 the formation of an amnion in the Pilidium constituting the only 

 important difference which can be pointed out between the modes of 

 origin of the young Nemertine in the two types. 



So far as is known the forms which develop in a Pilidium, or 

 according to the type of Desor, all belong to the division of the 

 Nemertines without stylets in the proboscis, known as the Anopla. 



Development without Metamorphosis. The majority of the 

 Nemertea, including the whole (?) of the Enopla, develope without 

 a metamorphosis. The observations which have been made on this 

 type are not very satisfactory, but appear to indicate that the for- 

 mation of the hypoblast may take place either by imagination or 

 by delamiuation. 



Invaginate types have been observed by Barrois (No. 192), Dieck (No. 

 196) and Hubrecht. 



Barrels' fullest observations were made on Amphiporus lactlfloreus (one 

 of the Enopla), and those of Dieck on Cejj/talot/trix galathece (one of the 

 Anopla). 



A regular segmentation is followed by a blastosphere stage with a 

 small segmentation cavity. In Barrois' type the inner ends of the cells of 

 the blastosphere are stated to fuse into a kind of syncytium. A small 

 imagination, takes place, and the cells which take part in it separate from the 

 epiblast, and then fuse with the syncytium within the blastosphere. Dieck 

 finds that in Cephalothrix the invaginated mass simply vanishes. 



Barrois' statements about the fusion of the syncytium derived from the 

 epiblast cells with the invaginated cells must be regarded as very doubtful. 

 The formation of the germinal layers takes place, according to Barrois, by 

 the separation of the internal mass of cells into mesoblast and hypoblast. 

 The proboscis is formed, according to this author, from the mesoblastic 

 tissues. Dieck, on the other hand, with greater probability, states that the 

 proboscis is formed by an invagination. In Cephalothrix a further point 

 deserves notice, in that the whole of the primitive epiblast becomes shed. 

 In this fact there may perhaps be recognised the last trace of a metamorphosis 

 like that in the type of Desor. 



Delaminate types have been studied by Barrois (No. 192) and Hoffman 

 (No. 198), both of whom give circumstantial accounts of their develop- 

 ment. 



Hoffman's account is especially deserving of attention, since his observa- 



