202 NEMERTEA. 



by a ciliated canal 1 . The proboscis arises as an hollow invagi- 

 nation 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, accord- 

 ing to Metschnikoff 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 Linens embryo within the 

 larval skin ; the formation of an amnion in the Pilidium consti- 

 tuting 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, develop 

 without a metamorphosis. The observations which have been 

 made on this type are not very satisfactory, but appear to 

 indicate that the formation of the hypoblast may take place 

 either by invagination or by delamination. 



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

 196) and Hubrecht. 



Barrois' fullest observations were made on Ainphiponts lactiflorens (one 

 of the Enopla), and those of Dieck on Cephalothrix galatkea (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 invagina- 

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



1 This is the view of both Metschnikoff (No. 202) and Leuckart and Pagenstecher 

 (No. 201), and is further confirmed by Barrois, but Butschli (No. 193), though he has 

 not observed the earliest stages of their outgrowth, believes them to be imaginations of 

 the Nemertine skin. 



