342 A^. M. Stevens and A. M. Boring. 



are no distinct layers. The apparent difference in the length 

 of the cilia in these two figures (4 and 5) may be due to their 

 being matted together in fixation. 



There is no definite ectoderm or endoderm. The cells com- 

 posing the mass of the body are the parenchyma cells, irregularly 

 spindle-shaped, with large nuclei. (Fig. 4, n.) In many places, 

 the outlines of these cells are so indefinite that it appears as though 

 they merged into one another, forming a syncytium studded here 

 and there with nuclei. Among these parenchyma cells are 

 mucous cells, which have similar nuclei, but contain masses of a 

 blue-staining secretion. (Fig. 4, m.) On the outer edge, where 

 one would expect to find a definite ectoderm, these parenchyma 

 cells are ciliated (Fig. 4, c), and stain a little more deeply, perhaps 

 due to a cuticular secretion; but in no other way is the outer layer 

 of cells different from the cells making up the mass of the body. 

 This outer layer is not even arranged regularly, for the nuclei are 

 at varying distances from the base of the cilia, and at irregular 

 distances apart. The cells of the digestive region (Fig. 2, d) — it 

 is not definite enough to be called a digestive tract— do not differ 

 in any respect from the other parenchyma cells. In places 

 pieces of crustaceans, which have been taken in as food, are found 

 in between the cells near the digestive region (Fig. 2, q), showing 

 that this cavity is continuous with the spaces between the loose 

 parenchyma cells. At the opening of the digestive region (Fig. 

 2, x), a few of the cells are sometimes ciliated (Fig. 2, c) like the 

 ectodermal parenchyma cells. 



Muscle fibers are scattered throughout the parenchyma, but 

 are accumulated especially among the ectodermal parenchyma 

 cells (Fig. 4, g), around the female reproductive opening, and in 

 the penis, of which they are the chief constituent. They vary 

 much in size, in fact, so much that in sections stained with iron 

 haematoxylin and orange G, some take the black and some the 

 yellow color. 



The reproductive cells are more distinctly differentiated than 

 the other cells in these flatworms. They are not grouped into 

 ovaries or testes, but they lie in definite positions among the paren- 

 chyma cells, and are discharged through definite openings, guarded 

 by muscle cells having a sphincter-like arrangement. The testis 

 cells (Fig. 2, t) extend along the lateral edge from near the anterior 

 end to the penis which is an external muscular organ. (Fig. 



