Pilidium of Certain N'emerteans. • 2 3 7 



tean. In 1869 Biltschli (6) published a further description of P. 

 gyrans and gave a detailed account of the development of the 

 nemertean. Salensky (28) has described very carefully the structure 

 of the mature pilidium, and the manner in which the nemertean devel- 

 ops from it. His observations on the structure of the pilidium 

 include good descriptions of the mesenchyme, the muscular system 

 and, more particularly, of the highly developed nervous system with 

 which he finds the embryo to be endowed. The development of the 

 nemertean from the pilidium is also described in detail and with 

 great clearness by Burger (4, 5). Wilson (33), Fewkes, and others 

 have published descriptions of several peculiar forms of pilidia. 

 Verrill (30) also figures two or more species without definite descrip- 

 tions. The development of the muscular system from the amoeboid 

 cells of the larval mesoderm has been described by C. B. Wilson (32) 

 for Cerebratulas lacteus. 



The only description of the development of the pilidium from 

 the egg, however, is that given by Metschnikoff (24^ 25) from 

 Linens {3Iicriira?) lacteus. The regular and equal cleavage of the 

 egg gives rise to a typical blastula with a comparatively large 

 segmentation cavity. The ventral cells (entoderm) soon become 

 columnar, while those on the dorsal surface (ectoderm) remain more 

 flattened. At the same time the cells of the whole blastula become 

 covered with cilia, and a large flagellum appears at the upper pole. 

 A regular invagination of the entoderm occurs after the separation 

 from it of a few mesoderm cells. The cavity of the invaginating 

 entoderm bends posteriorly to form the digestive canal. With the 

 appearance of lappets the gastrula can be recognized as a pilidium. 

 A series of infoldings of the walls of this pilidium gives rise to the 

 body of the nemertean. 



fertilization. 



A detailed account of the maturation and fertilization of the eggs 

 of C. marginatus has been published recently in the Zoologischen 

 Jahrbtichern (8). Soon after the eg^ comes in contact with the 

 water, a pair of very minute asters appear beside the nuclear mem- 

 brane, as in other eggs. Their ultimate origin was not determined. 

 A typical polar spindle is formed, and the sixteen ring-like chromo- 

 somes divide, whether the egg has been fertilized or not. The process 

 will go no further, however, until after fertilization, when the polar 

 bodies are formed as usual. The first polar body occasionally divides 

 into two. The egg is oriented even before deposition : for the asters 



