ß22 C. O. WHITMAN, 



briefly sketch. Its position is very remarkable, and we at once inquire 

 how it can be reconciled with the metamerism of the head. This ru- 

 dimentary eye does not belong to the same segment as the principal 

 eye (III), but to the segment in front (II). The development shows 

 this conclusively. The place of origin is at the surface, on the hind 

 margin of ring 2. The ectodermal cells multiply at this point, giving 

 rise to a mass of rounded or polygonal cells, which are all alike at 

 first. Later the mass difierentiates into two distinct kinds of cells, 

 1) peripheral hair-bearing cells and 2) deeper, large clear cells. The 

 former are probably tactile, the latter visual. 



While this difierentiation is going on, a portion of the visual 

 elements is carried back beneath the 2'^ pair of eyes, and here takes 

 the form of a small pigmented eye. The rest of the visual cells arrange 

 themselves beneath the tactile cells around the common nerve, and 

 remain throughout life without any pigment investment. 



This backward migration of the deeper portions of the first pair 

 of eyes seems to be due, in part at least, to lack of space for de- 

 velopment in segment II. Just beneath the point of origin, we meet 

 with a large cavity, representing the junction of the marginal and 

 median vascular sinuses. The organ is much larger than the space 

 above this cavity will accommodate, and its elongation carries it in the 

 direction of the 2^ pair of eyes." 



The Hirudo Eye. 



Maier brings out one fact that has hitherto escaped the attention 

 of other authors: namely, that the eye of Hirudo and Aulastomum 

 is supplied with two nerve-branches, one of which is axial while 

 the other passes up in front of the eye, innervating „the anterior 

 ventral cells" (p. 562, 575, 577). The significance of this pe- 

 culiarity completely escaped Mr. Maier, as might be expected from 

 one who not only failed to detect the leading features in the morpho- 

 logy of the eye, but also failed to recognize them years after they 

 had been pointed out and accepted by many authors. 



For example, the serial homology of the eyes and the meta- 

 meric sensillae was disputed (p. 574 — 575), and this radical error 

 naturally led to faulty observations and misinterpretations. 



The fact that visual cells are associated with tactile cells as 

 integral parts of the sensilla failed of recognition. 



The visual cells were supposed to be isolated cells scattered through 



