120 



DEVELOPMENT OP THE AJTOPLA. 



indistinct transverse rows. Each ovoid flask (Plate XXIII, fig. 11) has a process as in Lineus 

 gesserensis, but it is much smaller ; and in the same manner contains several yolks. Unfortunately, 

 from defective arrangements and the very hot weather, all the embryos were dead, only a little isolated 

 ciliation being observed on certain cells. The embryos were furnished with black eye-specks. 



In the remarkable development of the Nemertean from the Pylidium-fomi, as first described by 

 Krohn and the celebrated J. Miiller, afterwards by Busch, Gegenbaur, Leuckart, and Pagenstecher, 

 and recently by MetschnikofT, the phases mentioned in the foregoing pages are considerably 

 increased in complexity. E. MetschnikofT finds that in the egg of a whitish Lineus from Messina 

 the usual changes ensue after impregnation, resulting in the formation of a ciliated embryo, which 

 by-and-by assumes the shape of a Pylidium (woodcut, fig. 6, a), having a depression of the 



Pig. 6. 



&//1 



-PyZ^ww-developnient. (After Dr. E. Metschnikoff.) 

 A. — Young Pylidium on its escape from the egg ; s, the oral involution. 

 B. — Profile of a Pylidium, showing the early condition of the csecal stomach (s), with its cellulo- granular coating ; 



e l e 2 , the anterior and posterior pair of processes from which the future Nemertean is developed. 

 C. — The same Pylidium viewed from the under surface. 



D. — Profile of an older Pylidium with the processes developing, and after the appearance of the cephalic sacs (#). 

 E. — Another Pylidium showing the elongated leaves developed from the original processes, with the elementary 



amnion (am). 

 F. — A further stage in the process, the young Nemertean being now outlined ; rh, the proboscis. 



