Pilidiwm of Certain Nemerteans. 245 



states of contraction it would be difficult, especially in the earlier 

 stages, to determine to which of the two species a given individual 

 belonged. 



The pilidium of C lacteus is easily distinguished from the above 

 by its larger size and much more highly developed lappets. A 

 detailed description of this species is promised in a forthcoming 

 paper by Prof. C. B. Wilson, who has already published a single 

 figure of it (32). 



The Pilidium of Cerebratulus niargiiiatus. 



At Naples, in 1896, the eggs of this species could be artificially- 

 fertilized during the latter part of March and throughout the month 

 of April. At the end of seven days the pilidium (Fig. 5, PI. xxxv) 

 is about .21^^ in length, and .21™™ from the apical plate to the lower 

 edge of the moderately extended lappet. It is therefore nearly 

 seven times as lai'ge as the pilidium of M. cceca or C leidyi of the 

 same age. The eggs themselves are also much larger and contain a 

 large amount of yolk. Development is consequently much slower 

 than in the two other species, the eggs of which contain but a small 

 quantity of yolk. The general course of development, however, is 

 similar in all. 



The segmentation cavity in the blastula is smaller than in M. cceca 

 and C. leidyi. Instead of being flattened, the blastula is much 

 higher than broad* The cells on the under sui'face become remarka- 

 bly long and encroach greatly upon the segmentation cavity (Fig. 3, 

 PI. xxxiii). Preparatory to invagination the entoderm cells become 

 so closely pressed together at their lower ends that they gradually 

 assume a conical or pear-shaped form, with their larger ends project- 

 ing far into the segmentation cavity. As the process goes on some 

 of them become pushed inward, separated entirely from the neigh- 

 boring cells, and float freely in the fluid of the segmentation cavity. 

 They thus become mesoderm-cells, and apparently supplement the 

 few primary mesoderm-cells which seem to have an origin similar to 

 that of the mesoderm in the annelids. 



One of these primary mesoderm-cells is seen in Fig. 3, PI. xxxiii, m, 

 in its original position at the lower, posterior border of the blastula. 

 Three other mesoderm cells are shown in red above the entoderm. 

 To the left of the primary mesoderm-cell are seen two very small 



*The gastrula of this species ia figured by Burger (.5, Figs. 1, 2, Taf. 30) from 

 sketches by Hubrecht. 



