No. 2.] COMPOUND EYES OF ANNELIDS. 295 



mum in Potamilla, where it is difficult to decide if the granules 

 are really in the cell, in neighboring pigment cells, or also in 

 some intercellular matrix. 



These compound eyes are found to be too simple in structure 

 to admit of any direct comparison with the eyes of Arthropods. 

 If the eye of Hypsicomus may be homologized with one element 

 of Sabella, as above suggested, that is, if we assume the forma- 

 tion of an elongated, cuticular, lens-like body by the gradual sink- 

 ing down of an epidermal cell, we would have a method of forma- 

 tion different from those open, pit-like depressions of many cells 

 assumed in the explanation of the Arthropod ommatideum. 



When compared with certain members of the Mollusca, how- 

 ever, we find quite close agreement. The peculiar, protuberant 

 mantle organs of the Lamellibranchs Area and Pectunculus 

 have quite the same structure as in these Annelids, as may be 

 seen from the figures of Rawitz (23), Carriere (22), and Patten 

 (24). However, the sense cell or refracting cell in these mol- 

 luscs is quite different from the Annelid cells represented in 

 Figs. 1 and 2 above. The refractive cuticular body has no such 

 shape as found in the Annelids, the nucleus of the cell is in its 

 outer part, there appears to be no specialized axial region exter- 

 nal to the nucleus/ but a peculiar conical body occupies the apical 

 part of the cell. 



This sensory cell is also not pigmented, thus agreeing with 

 the sensory cell of Branchiomma as described by Brunotte. 



Though this similarity is too general to point to definite mor- 

 phological conclusions, being merely the agreement of pig- 

 mented epithelial regions having sense cells surrounded by 

 pigment cells ; yet taken with the fact of linear arrangement 

 of the organs, of their occurrence in large numbers, and of the 

 like responses of the animals to light, it will, I think, furnish 

 evidence as to these functions in both animals. 



In both cases the animals respond very quickly to slight 

 sudden changes in the intensity of illumination, bivalves seeking 

 safety by retreat within the hard shell, the Annelid withdrawing 

 into firm tubes. 



Though the structure of these organs is such as to justify our 

 interpretation of them as undoubted eyes in the common mor- 

 phological sense, yet the following considerations show the need 

 of physiological experimentation here. 



