No. 2.] COMPOUND EYES OF ANNELIDS. 293 



and the refracting cone. Along with this retreat of the sensory 

 or refracting cell, away from the surface, there has taken place 

 a bending, and also a sinking in, of the surrounding pigment 

 cells. 



If this comparison be correct, the eye of Hypsicomus has 

 much in common with a single element or ommatideum-like 

 collection of cells in the compound eyes of the other Annelids 

 examined ; at the same time it presents interesting resemblances 

 to the common eyes found upon the heads of higher Annelids, 

 as I hope to illustrate in another paper. 



The clear spherule compared above to the shortened axial 

 region of the refracting cells of Sabella, etc., also suggests 

 the phasospheres of certain scorpions. 



A review of the literature shows that eyes such as those 

 described in the present paper are widely distributed through 

 the Sabellidse and Serpulidse. Excepting a few doubtful organs 

 that may prove to have structure and function different from 

 that of true branchial eyes, all but one of the organs referred to 

 in the historical review may be readily reduced, either to the 

 type of Potamilla or to that of Hypsicomus. Most of the obser- 

 vations are, however, of too superficial a character to admit of 

 detailed comparisons as regards cellular structure. 



The one exceptional organ, the eye of a species of Branchi- 

 omma, is very important, since it is about the only one that 

 has been studied by the aid of sections and depigmentation. 

 Although the general anatomy of this eye is identical with that 

 of Sabella, etc., yet the arrangement of the cells, according to 

 Brunotte ( 8, 9), is fundamentally different. While the eyes of 

 the Annelids described in this present paper are composed of a 

 single layer of epithelium, those of Branchiomma are of two 

 layers, each element having an outer corneagen cell and an 

 inner sensory cell, both surrounded by small pigment cells. 

 There is, moreover, no pigment in the sensory cells. 



If, however, it were permissible to interpret the figures given 

 in Brunotte's monograph (9) in the light of experience gained 

 in studying Potamilla and Sabella, one could readily explain 

 them as being representatives of what is sometimes observed 

 in these latter animals, but not representing the true structure. 

 Thus if sections are cut obliquely, or not thin enough to prevent 

 superposition of cells, or if strong nitric acid produces such 



