1007. J NATURAL SCIENCES OF IMIILADKLIMII A. 95- 



posterior part of the last segment, especially on the dorsal side, the 

 hypoderniis is also much thickened. On the dorsum and sides of the 

 trunk, however, the thickness of the body wall is closely related to the 

 external segmentation. At the middle region of each segment the 

 hypoderniis reaches the thickness attained by its ventral portion 

 (5-S /O ; it gradually becomes thinner toward the limits of each segment, 

 its minimum, ca. 3 /«, being reached in the intersegmental regions. 

 These differences in the height of the hypodermal cells find expression 

 on the external surface of the body wall alone, which consequently in 

 longitudinal section presents an undulating outline. In cases where 

 the contraction of the animal has been excessive, the exterior of the 

 body wall may even have the appearance of being deeply indented in 

 the intersegmental regions. 



Supporting Cells. — The hypodermal cells are differentiated into sup- 

 porting cells, ciliated and non-ciliated, and gland cells. The non- 

 ciliated supporting cells (fig. 14, also figs. 2-8), which make up the 

 greater portion of the hypodermis, are simple epithelial cells, flat in the 

 thinner portion of the body wall, but in the thicker portions becoming 

 cuboidal and even columnar, as in some parts of the head. In the 

 ventral portion of the latter, for example, the hypodermal cells are so 

 closely set together that the bases of some of them seem to have been 

 forced inward, and have the appearance of pyriform processes, each of 

 which contains a nucleus (PI. XIII, fig. 23). The cytoplasm of the non- 

 ciliated cells is clear; high magnification presents an alveolar appear- 

 ance. Throughout the region of the head, next to the hypodermis, is 

 a distinct layer of vacuoles (figs. 2 and 12), similar to that described by 

 Meyer (1901) as occurring in the larva of Lopadorhynchus. A layer of 

 smaller vacuoles is also discernible in the hypodermis of the trunk (fig. 

 6). The nuclei are spherical or ovoid, provided with an open reticulum 

 of linin, bearing minute chromatin granules and a large and prominent 

 chromatin nucleolus (karyosome). The ciliated cells may, in sections, 

 be at once distinguished by their darker aspect, and are, as described 

 in the preceding section, distributed to form two ventral tracts and nine 

 bands. The latter are, with the exception of the second cephalic 

 band, composed of but one row of cells, their outer surfaces elongated 

 in the direction of the band (fig. 27), and meeting onl}^ at their extreme 

 ends. The second cephalic ring or band is, on the other hand, com- 

 posed of two rows of cells (fig. 28, c.r. 2), similar to those of the other 

 bands. This difference has also been noted by Schimkewitsch (1895) 

 in the White Sea Dinophilus {D. vorticoides) . The cells composing 

 the cephalic bands are wedge-shaped, in conformity with the general 



