NEMERTIXI. 



341 



cocoon. Some forms, as Prosorochmus Claparedii and Tetrastemma 

 obscurum, are viviparous. 



Some of the Anopla develop with a metamorphosis. The larva is 

 ciliated and 

 may pass 

 through a 

 free - swimming 

 stage, in which 

 case it is known 

 as the Pili- 

 dium, or it 

 may be without 

 such a stage 

 (TypeofDesor). 

 In both cases 

 the perfect 

 worm is deve- 

 loped within 

 the skin of the 

 ciliated larva. 



FIG. 277. Pilidium (after E. Metschnikoff). a, free swimming larva 

 with invaginated cavity ; b, later stage, helmet-shaped ; E, E 1 the 

 two pairs of ectodermal imaginations ; D, alimentary canal. 



The Pilidium larva is helmet-shaped, and was formerly described as 

 the species of a supposed independent genus, Pilidium, and presents 



many analogies to the 

 Echinoderm larva. In the 

 case of the Pilidium, the 

 segmentation is regular, 

 and results in the formation 

 of a spherical ciliated em- 

 bryo, which is hatched and 

 becomes a free-swimming 

 larva; the archenteron is 

 then formed by invagina- 

 tion ; and at the side of the 

 embryo, opposite the blasto- 

 pore, a long flagellum is 

 developed (fig. 277, a). On 

 each side of the mouth a 

 broad lobe grows out, the 

 edges of which are fringed 

 with cilia (fig. 277, b). 

 Two pairs of invagmations of the ectoderm now make their appear- 



FIG. 278. Later stage of Pilidium, with tuft of cilia 

 and enclosed Nemertine (after Butschli); Oe, 

 oesophagus; D, alimentary canal ; Am, amnion; 

 H, rudimentary proboscis of the Nemertine ; So, 

 lateral pit. 



