154 BULLETIN OF THE 



finally obscure the reticulum, and impart to the entire cell, save its 

 marginal zone, an appearance almost opaque (Fig. 7). 'J'he secretion 

 first appears in the vacuole in the foi'm of minute beads at the periph- 

 eral ends of the reticular fibrils which traverse the clear zone and 

 terminate at the edge of the vacuole each in a single bead (Fig. 7). 

 During the formation of the secretion in the vacuole, the mass of 

 opaque granules moves toward this space ; and the close of the process is 

 represented in Figure 8, where the vacuole is filled with a homogeneous 

 mass, displaying in a somewhat lesser degree the affinity for hema- 

 toxylin stains which characterized the granules while contained in the 

 cell substance itself. At the same time, these granules have disappeared, 

 except a few which are grouped in a zone about the vacuole ; and the 

 cell has become thereby so much lighter 9,s to show the reticulum at its 

 proximal end. 



This description of the activity of these organs would seem to place 

 their glandular nature beyond question. In comparing the two sorts of 

 glands, it is of great importance to note that the cells do not show in 

 this case any connection with nerves, whereby they are sharply distin- 

 guished from the multicellular glands. The space (Spalt) which Andreae 

 ('81, p. 215) describes as existing between the cells of these glands was 

 found not infrequently in some preparations, but it is evidently due to 

 shrinkage. The double membrane separating the cells, described by the 

 same author, was probably produced ui the same way. 



The distribution of these glands is peculiar. Over the general sur- 

 face of the body they are found only rarely, and on the introvert they 

 are present only in the pajnllce, the interspaces being entirely fi-ee from 

 them. Each papilla of the posterior zone of the introvert shows in 

 surface views an irregular double or triple row crossing the convex outer 

 surface near the base, and occupying one half to one third of its entire 

 breadth. Rarely isolated bicellular glands are found near the tip. This 

 regular limited distribution allows perhaps a conjecture as to their 

 possible function. Inasmuch as the behavior of the secretion toward 

 coloring reagents would seem to mark it as mucine (cf. Hoyer, '90), may 

 it not be that these glands furnish the lubricant demanded by the con- 

 stant movements of the two walls of the introvert 1 The papillae are 

 especially affected, of course, rubbing against each other in the con- 

 stant inversion and eversion. They receive, furthermore, the greater 

 part of the pressure as the animal forces its way through the sand, 

 in the method described by Andrews ('90, p. 391). The animal does 

 not advance backward with the " Eichel voran," as maintained by An- 



