236 W. R. Coe — Development of the 



The indirect type, in which there is a pilidium formed, is known 

 in still fewer species, and these belong to the Ileteroneraerteans, in 

 most of which cephalic slits are developed, Metschnikoff (25) found 

 that this type occurs in Linens lacteus of Europe. There have like- 

 wise been several species of pilidium described from Europe, and 

 three or more from America, but with one or two exceptions it has 

 never been known to what species of nemerteans they belonged, 

 Joh, Miiller as early as 1854 (26) found that Pilidium gyrans gave 

 rise to a young nemertean which he considered identical with 

 Micrura fasciolata. 



C, B, Wilson (32) has raised the young pilidium from the eggs of 

 (Jerehratulus lacteus. The young pilidium of this same species has 

 been known to me for several years. The pilidia of Cerebratulus 

 leidyi and of Micrura cceca* may easily be reared from the eggs 

 during the months of July and August, It is from these two species, 

 as well as from Cerebratulus marginatus., from Naples, that material 

 for the present paper has been obtained. 



The deposition of the eggs and the early development of Lineus 

 viridis were well described by Desor (10) in 1848 under the name of 

 Nemertes ohscura. The ciliated larva which he describes, and which 

 represents an intermediate stage between the nemerteans with direct 

 development and those in which a pilidium is formed, has since been 

 known as Desor's larva. The complete embryology of this species 

 has now become well known through the researches of M. Schultze 

 (29), V. Beneden (3), Macintosh (22), Barrois (2), Hubrecht (16), 

 and Arnold (1). Btirger, in his splendid monograph on the nemer- 

 teans (5), gives a clear summary of both the direct and indirect types 

 of development. 



On the development of the pilidium very little has yet been pub- 

 lished, although several authors have described more or less fully 

 the development of the young nemertean within the pilidium. Joh. 

 Mtlller in 1847 gave a good description of a pilidium which he recog- 

 nized as being a larval form of some animal, and which he named 

 P. gyrans. In 1854 (26), after further investigation, he discovered 

 that a young nemertean developed within the pilidium. This nemer- 

 tean he supposed to be the young of M. fasciolata. Gegenbaur in 

 1854 (12) and Krohn in 1858 (19) discuss further the relations of 

 the young nemertean to the pilidium. Leuckart and Pagenstecher 

 (21) describe the structure of P. gyrans and P. auricidatum, and 

 give brief notes in regard to the development of the young nemer- 



* Yerrill (31). 



