STUDIES ON THE PROTOOHORDATA. 297 



the form of a number of pigmented granules which are deposited 

 in the interior of certain cells of the dorsal wall of the cerebral 

 portion of the medullary tube. Without entering into a de- 

 tailed histological account of the sense-organs I will confine 

 myself to a few points in which I can to a certain extent sup- 

 plement the classical work ofKowalevsky. 



The origin of the sense-organs in point of time seems to 

 underlie a certain amount of variation. Kowalevsky describes 

 the otolith as arising first in Phallusia mammillata. In 

 the embryo from which fig. 1 was drawn the eye was the first 

 to appear, being represented at this early stage by scattered 

 rounded pigment granules lying in several — four or five — of 

 the cells in the dorsal wall of the cerebral vesicle, while in 

 fig. 3 the otolith and eye appear simultaneously. The otolith, 

 as well as the eye, first appears in the form of a number of 

 scattered pigment granules very similar to those which go to 

 form the eye, but rather larger, and difi'ering from the latter 

 in their being confined to one cell, while, as I have just men- 

 tioned, the pigment granules of the eye are distributed among 

 several cells. Kowalevsky would appear not to have seen the 

 eye granules at their very first origin. He says (loc. cit., 

 p. 117), '^ Am Grunde der Zellen des hinteren abgesetzten 

 Theils der Blase [i.e. cerebral vesicle] erscheinen sehr feine 

 Pigmentkorner," and he figures them (Taf. xii, fig. 31) outside 

 the cells which form them. It is possible that this may be 

 their mode of deposition in the species studied by Kowalevsky — 

 viz. Phallusia mammillata. 



The granules which belong to the eye have at first essentially 

 the same character and nearly the same size as those which 

 go to form the otolith, being scattered throughout the body of 

 the cells in which they lie (fig. 1); but as they increase in 

 number they become very much smaller, and then lie entirely 

 at the inner free extremities of the cells (figs. 3 — 5). The 

 otolith granules do not tend to increase in number, but retain 

 their original size until they fuse together (fig. 5) . 



The otolith cell or otocyst lies immediately in front of and 

 adjacent to the eye-cells, and, in fact, forms primarily one of 



