Pilidimn of Certain Nemerteans. 261 



Figure 2. — Dorsal view of pilidium of M. cceca at the age of twelve days. In the 

 center are the cells of the apical plate (black) with two large flagella directed 

 upwards. To the right and left of the apical plate are seven or eight muscle- 

 cells with their fibers running down the sides of the body towards the lappets. 

 These fibers constitute the lateral muscular band. In front and behind the 

 apical plate the muscle-cells are few in number, and repeatedly anastomose. The 

 ectoderm is not shown ; a, anterior, p, posterior end. Slightly diagrammatic. 

 x500. 



Figure H. — Pilidium of M. cceca at the age of twelve days seen from the left aide. 

 The epithelium is supposed to be transparent except at the margins of the draw- 

 ing, where it is shown in section to illustrate its thickness ; a, anterior end, p, pos- 

 terior end. At the side of the apical plate are seen six muscle-cells with nearly 

 parallel fibers running directly into the lappet. These lie immediately beneath 

 the surface epithelium, and constitute the lateral muscular band. A few cells on 

 the lateral face of the embryo send out anastomosing fibers in all directions. 

 The horizontal band of muscles, indicated by the dotted lines (6), are those which 

 run antero-posteriorly at the sides of the mouth. Three mesenchyme cells are 

 indicated by c, while a half-dozen others are seen between the muscle-cells in 

 the lappets. Slightly diagrammatic. The number and position of the muscle- 

 cells and fibers was confirmed in the living object, and also in many stained 

 preparations, x 500. 



Figure 4. — An anterior view of a pilidium of the same age as those in figs, 2 and 3. 



Figure 5.— Pilidium of C. marginatus at the age of ten days. The dotted lines indi- 

 cate the digestive tract. 



Figure 6. — Section through the apical plate to show the relation of the muscle-cells 

 to the epithelium. 



LITERATURE. 



1. Arnold, G., Zur Entwicklungsgeschichte des Linens gesserensis. Trav. Soc. Imp. 



Nat. St. Petersbourg, vol. xxviii, 1898. Review by 0. Burger in Zool. Cen- 

 tralblatt, voL vi, 1899. 



2. Barrois, J., Memoire sur I'Embryologie des Nemerles. Anuals des Sci. Nat., 



T. vi, 1877. 



3. van Beneden, P. J., Recherches sur la faune littorale de Belgique. Mem. Acad. 



Sci. de Belgique, T. xxxii, 1861. 



4. Biirger, 0., Studien zu einer Revision der Entwicklungsgeschichte der Nemer- 



tmen. Ber. Nat. Ges. Freiburg, Bd. viii, 1894. 



5. Nemertinen. Monograph 22, Fauna und Flora des Golfes von Neapel. 



Berlin, 1895. 



6. Biitschli, 0., Einige Bemerkungen zur Metamorphose des Pilidiums. Archiv. 



f. Naturgeschichte, 1873. 



7. Child, 0. M., The maturation and fertilization of the egg of Arenicola marina. 



Trans. N. Y. Acad. Sci., vol. xvi, 1897. 



8. Coe, TV. R., The maturation and fertilization of ihe egg of Cerebratulus. Zool. 



Jahrbiicher, Bd. xii, 1899. 



9. Conklin, E. G., The Embryology of Crepidula. Journ. Morpholog^v, vol. xiii, 1897. 

 10. Desor, E., Embryology of Nemertes, etc. Boston Journ. Nat. Hist., vol. vi, 1848. 



