Pilidium of Certain Nemerteans. 243 



later disappear. By the flattening of the cells the size of the embryo . 

 is greatly increased, measuring about .11™™ in diameter, and the 

 ectoderm is more widely separated from the enteron (Fig. 2, PI. 

 xxxiii). At the ^me time the ectoderm at the two lower, lateral 

 edges of the embryo extends downward to form the side-lobes, or 

 lappets, of the pilidium. With the further growth of the pilidium 

 the lappets become more or less highly developed, according to the 

 peculiarities of the particular species to which it belongs (Figs. 5, 6, 

 PI. xxxii). 



The cells at the extreme upper pole of the embryo, instead of 

 flattening out, become more crowded and columnar, and form the 

 apical plate (Figs. 1, 2, PI. xxxiii). This plate is at first covered 

 with numerous long cilia which continue to increase in length and 

 gradually fuse together to form a few, very long flagella. As the 

 pilidium grows older this fusion goes on until in many cases only a 

 single large flagellum remains. 



At the end of the first day the digestive tract (Fig. 2, PI. xxxiii) 

 becomes difiierentiated into two distinct cavities— the esophagus and 

 the intestine, the peculiarities of which will be described in detail 

 below. 



The Pilidium of 3Iicrura cceca. 



At the age of six or eight days the pilidium of this species meas- 

 ures about .14-.15"™ in length, and just about the same in a vertical 

 direction from the apical plate to the bottom of the lappets. The 

 pilidia usually swim near the surface of the water, with the flagellum 

 directed upwards. A large proportion of all those contained in a 

 vessel are usually collected in a single small space which may, or 

 may not, be on the side of the vessel turned toward the light. 



Like other pelagic embryos the pilidium is almost perfectly trans- 

 parent. In general form it is balloon- or helmet- shaped with pecu- 

 liarly short lappets (Fig. 6, Pi. xxxn). This pilidium differs from 

 most other species in being of much greater diameter a short dis- 

 tance below the apical plate than it is on a level with the mouth. 

 Seen from above, the outline of the body is oval or elliptical, with 

 the longer diameter directed antero-posteriorly. The dorsal and 

 lateral surfaces (which make up the so-called umbrellar surface) 

 are formed from very thin five- or six-sided cells, each with a small, 

 disk-shaped nucleus scarcely more than one-eighth the diameter of 

 the cell. There are not usually more than sixty to eighty of these 

 cells on the whole umbrellar surface exclusive of the lappets. Their 



