"V. — On the Development of the Pilidium of Certain 

 Nemekteans. By Wesley R. Coe. 



As is well known, the embryology of the nemerteans presents two 

 distinct types of development — the indirect and the direct. In the 

 indirect type the cleavage of the egg results in the formation of a 

 free-swimming larva of complex structure known as the jDilidium. 

 Invaginations of the body-walls of this pilidium give rise to a young 

 nemertean possessing the form of the adult. In the direct type there 

 is no such differentiated larva, the segmentation of the egg resulting 

 directly in the formation of a worm resembling the parents. 



Intermediate between these two extreme types is a third form 

 where there is a comparatively thin external, ciliated skin, inside 

 which the young worm attains a rapid development to the adult 

 form. Such an intermediate form is as yet known only in a single 

 species of Lineics (X. viridis =z gesserensis) common both on the 

 shores of New England and in Europe. These larvfe are known as 

 Desor's larvae. 



The direct type has been found to occur both in the Mesonemer- 

 teans and in Metanemerteans, and from its wide distribution we may 

 conclude that it is common to a much greater number of species than 

 is the indirect type. In a portion of these, however, the outer ciliated 

 layer of ectoderm is thrown off (as in haany other Platyhelminthsj 

 after a second ciliated layer has formed beneath it. This occurs in 

 Cephalothrix galathece, according to Dieck (11). Burger (5) is of 

 the opinion that this is likewise true in Prosorochnms. In other 

 species the outer covering of the larva, as Salensky (27) found in 

 Monopora, passes directly into that of the adult. In only a few 

 species of nemerteans, however, have we any knowledge whatever 

 in regard to the embryology. Of these the development is but very 

 superficially known in all except a half dozen species. Salensky has 

 described the direct development of Monopora vivipara (27). The 

 same type of development has been studied by Burger (5) in Pros- 

 orochnms, which is likewise a viviparous species. In addition to 

 these Lebedinsky (20) has recently given a detailed account of the 

 direct mode of development in Drepanophorus spectahilis and Tetras- 

 temnia vermicidus. 



Among the nemerteans of New England I have noticed that the 

 direct type of development occurs in Amphiporus oohraceiis, A. 

 virescens and Cephalothrix linearis. 



