﻿OF THE ACALEPHiE OF NORTH AMERICA. 269 



lower margin of the bulb at the base of the shorter outer tentacles, where they are 

 smallest and nearest the chymiferous tube. They gradually recede, however, more and 

 more from the base of the tentacles, rising upon a slight prominence of their lower sur- 

 face, and growing larger and larger, till we observe the largest at the base of the longest 

 of the middle tentacles, with a very prominent black speck. When contracted laterally 

 (Plate II. Fig. 15), as constantly occurs when the animal shuts its lower opening by the 

 contraction of the inferior muscular partition, the bulb is folded up, and then divides into 

 three unequal masses, the middle folds following the margin of the longest tentacle, and 

 the two lateral masses answering to two bunches of lateral tentacles. When, on the con- 

 trary, it is fully spread out, but seen from the edge, as in Plate II. Fig. 13, it is almost 

 triangular, or rather flat crescent-shaped, its surface is smooth, and the specks are seen 

 through the tentacles. From these observations, it must be plain that the bulb is not hol- 

 low, but that it consists of a mass of dense cells arranged in a particular way. A triangular 

 dilatation (Plate II. Fig. 14, d, d) at the base inwards consists more of sensitive cells, upon 

 which are arranged, in a somewhat semicircular disposition, small, dark pigment-cells, 

 grouped together towards the periphery to form prominent cones, the points of which 

 are surrounded by more transparent parenchymatous cells, which form the base of the ten- 

 tacles. The eye-specks at the base of the tentacles are not directly connected with the 

 pigment-cells of the bulb ; they form more compact and darker dots by themselves, in 

 little prominences of the lower surface of the base of the tentacles, as seen in Plate II. 

 Fig. 1, a. The accumulations of pigment-cells, which constitute the eye-specks, appear 

 either as spherical or as circular disks, and assume at other times an ovate shape, or even 

 the form of crescents. These various appearances must be the consequence of the ar- 

 rangement of the cells, for they are always more or less circular, when seen from above 

 or through the tentacles (Plate II. Fig. 13), and they assume a more crescent-shaped 

 form when the tentacles are raised and the eye-specks are seen from the edge. I conceive, 

 therefore, that they have the form of watch-glasses, lining the sac-like projection of the 

 base of the tentacles, and that the crescent-shaped appearance arises either from the 

 curve of the projection at the base of the tentacles, or from the circumstance of their 

 being seen more or less in profile, or from both circumstances combined. But one thing 

 is plain, — and a very important circumstance it is, — that the pigment-cones (Plate II. 

 Fig. 12, 14, 16) of the bulb point to the centre of the eye-specks, which shows a close con- 

 nection between these dark dots and the centre of the bulb where the nervous ganglion is 

 seated ; and though this is not an arrangement known in the organs of vision of any 

 other animals, we are at least reminded by these peculiarities of the structure of 

 the compound eyes of insects, in which the pigment-pillars intervening between the 



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