MUSEUM OF COMPARATIVE ZOOLOGY. 171 



is as if we had a hood on the oral side of the sense organ only split along the 

 medial line. 



Into these curtains extend the dendritic branches from the sides of the main 

 branches of the extensions from the stomach into the sense octants of the um- 

 brella. What the " hood " accomplishes on the aboral side, these lappets par- 

 tially perform on the oral aspect of the disk. Both structures serve for the 

 protection of the delicate parts which they surround. 



Upon the base of the style which bears the otocyst in the genus Cyanea, and 

 on its oral surface, there rises an elevation, Plate VII. fig. 14, covered with 

 papilL-B, which I think are connected with the function of sensation. This 

 structure takes the place of an organ found on the aboral surface of the disk of 

 Aurelia, and called by Eimer the " Sinnespolster." The " Sinnespolster " of 

 Aurelia, as he says, is wanting in the aboral surface of Cyanea. It is repre- 

 sented in part by this wart-like protuberance covered with papillae (Plate VII. 

 figs. 5, 14), on the under surface of the otocyst style. Whatever the function 

 of this organ may be, it has escaped the notice of all those who have studied the 

 nervous and sensatory systems of this genus. In Aurelia this protuberance on 

 the style in Cyanea is wholly wanting, and is perhaps represented by that struc- 

 ture wanting in Cyanea, and called the " Sinnespolster." 



There remains yet to be noticed in my description of the general form of the 

 sense or^an of Cyanea certain hollows or angles in the neighborhood of the oto- 

 cysts formed in the rim of the umbrella. I refer to organs which Eimer has 

 called the inner " Reichgriibschen." As the style of the otocyst rises from the 

 margin of the umbrella, it leaves on either side, between it and the lateral folds, 

 two small recesses. One of these cavities is shown on Plate VII. fig. 12, just 

 above the point where the curtains, c, begin to rise from the oral surface of the 

 sense lappets, and on a level marked by a line drawn through the papillae per- 

 pendicular to the radius of the medusa. On the aboral side the " hood," and 

 on the lower, which is for the most part open, a part of the ends of oral cur- 

 tains (c), Fig. 12, enclose the recesses thus left, so that they resemble imper- 

 fectly closed furrows in the edge of the disk walls. These cavities are said 

 to be sensitive, their walls are so thickly set with nerve cells. The cavity of 

 the adult otocyst is filled with hexagonal calcareous otoliths of prismatic shape, 

 terminated by six-sided pyramids. The centre of each prism is filled with a 

 small cube which resists the action of caustic potash. In addition to an elon- 

 gated prismatic form with terminal pyramids, many of these otoliths are hex- 

 agonal lozenge-shaped, with flat terminal facets. The color of the otoliths is 

 bright orange, and when enclosed in the otocyst renders it very prominent in 

 the midst of the transparent walls of the umbrella. A cluster of small otoliths 

 is found on the under side of the otocyst near its junction with the style, wliich 

 may be the same as the ocellus described by Claus in Aurelia, while the larger 

 otoliths belong more to a different organ of sensation. If that is true, in the 

 otocyst of the Cyanea is an organ of sense representing the ocellus and the true 

 otocyst of certain hydroid medusae. 



The walls of the otocyst are made up of three layers, of which the external 



