1883.] NATURAL SCIENCES OP PHILADELPHIA. 235 



the upper lip, exactly in that part which, according to Moquin- 

 Tandon, was used for feeling. 



The position and presence of this organ can best be demon- 

 strated in longitudinal sections of A. lacustris (PI. X, tig. 9), as 

 in this species it is better developed than in A. Jiuviatilis. 



This organ is made up of a certain number of specialized 

 epithelial cells, which are connected with the cerebral ganglion by 

 fine nerves ; there are two organs which make a pair, and form a 

 patch on each side of the median line of the upper lip, and each 

 is connected with the cerebral ganglion of its own side. 



The cells which make up this organ differ principally from the 

 surrounding epidermal cells in their great size (PI. X, fig. 9 6-c\ 

 These specialized cells are not all of the same size, those in the 

 centre of the patch being the longer; and as we approach the 

 periphery, they grow smaller and smaller, until they pass imper- 

 ceptibly into the surrounding epidermis. This can be seen in the 

 drawing (PI. X, fig. 9), which represents a longitudinal section 

 through the upper lip of A. lacuatria. 



The external or free surface of these cells is covered with long 

 cilia, which thus differ from the cilia of the surrounding epithe- 

 lium. The nuclei of these cylindrical cells differ from those 

 found in the neighboring epithelium in form as well as in size. 

 When the object is colored in picro-carmine, the nuclei take a deep 

 color, and stand out sharply from the rest of the cell. Although 

 these nuclei are somewhat different among themselves, they are. in 

 general, fusiform. In this respect they differ from the regular, 

 oval-shaped nuclei of the epidermis. Some of these nuclei appear 

 bent, while others are straight. In fig, 9 (PI. X) we see that 

 some of the nuclei are pointed only at one end, and others at the 

 other, while only one is pointed at botli. In reality, all the nuclei 

 are pointed at both ends, and the reason that they are not so in 

 the drawing is that the nuclei have been cut in two, the knife not 

 happening to pass from one point to the other, but to have taken 

 an oblique course. In consequence of this, some represent the 

 one half, and others the other half, of the nucleus. The bending of 

 tlfe nuclei is due, I believe, to action of the re-agents used in 

 preparing the specimen. 



The substance of the nuclei is granular, as the other epidermal 

 nuclei, and I could not find the existence of a nucleolus. 



The nerve-endings, which enter the cells of this organ, are the 



