MUSEUM OF COMPARATIVE ZOOLOGY. 157 



crowding of the otherwise scattered multicellular glands in the centre 

 of some interspace of more than average size. Then the bicellular glands 

 make their appearance as a loose double row, and so quickly that no 

 intermediate stage could be found. They grow more crowded, and soon 

 after their appearance a shallow furrow may be seen to enclose the 

 mammiform area which they occupy. The skin seems to be tucked in 

 on the three sides at once, and as the furrow grows deeper the papilla 

 becomes more and more prominent. The growth in any papilla is in- 

 crease in breadth rather than in length, so that the relative dimensions 

 gradually change, and the older papillae in any region are markedly 

 wider than those more recently formed, while the length remains nearly 

 constant throughout the entire zone. 



Sense Papillce. — The papillae of the anterior zone are thickenings or 

 modifications of the hypodermis, rather than typical papillae like the 

 posterior ones ; they correspond probably to the " Wimperdriisen " of 

 Yogt und Yung ('88, p. 406). They are externally marked as small 

 rounded prominences of the skin, varying in diameter from .15 to .40 mm., 

 and often exhibit an oval or dumbbell-shaped opening in the centre of the 

 prominence. Viewed in cross section (Plate II. Fig. 18) they display an 

 evenly rounded contour, which is surmounted by cilia. These are short 

 on the lateral margins of the area, but increase in length as they approach 

 the apex, where they are longest. If one notices the basement mem- 

 brane, here for the first time well developed, it will be seen that the 

 prominence is almost entirely due to the increased height of the hypo- 

 dermal cells, which have changed their form from that of the usual hypo- 

 dermal elements so as to assume the character of filamentous cells, such 

 as compose the hypoderm of the tentacles, with which they are identical. 

 The isolated elements of the latter (Plate II. Fig. 21) might, indeed, an- 

 swer equally well as types of these cells. In addition to the elongated 

 nuclei of these cells, some few rounded ones are seen scattered between the 

 filamentous cells, more usually near the basement membrane. Perhaps 

 more common than the normal expanded form of the papillae, just de- 

 scribed, is the retracted condition shown in Figure 17. Such are found 

 in all degrees of contraction, alternating iiTcgularly with the normal form. 

 The papilla figured is perhaps fully retracted, and one notes that the ap- 

 ical area lies sunk in the structure, so as to give the effect of a cavity and 

 a duct. That this is due in part to the contraction of the cells themselves, 

 and in part to the retraction of the central portion of the papilla, is clear 

 from a comparison of Figures 17 and 18. In spite of this, I was unable 

 to identify any muscular elements connected with the organ, the many 



