THE CHROMATOPHORES OP ANIMALS. 117 



I would place the chromatophores of Crustacea in the same 

 category as these branched cells of the Insectan epidermis, 

 and would draw attention to the fact, established by Max 

 Weber, that in cavernicolous forms the chromatophores of 

 the ectoderm are unpigmented, in which condition they 

 closely resemble the branched ectodermal cells of insects. 



The Chromatophores of Mollusca. So much attention 

 has been directed to the study of the pigment-cell in Cepha- 

 lopoda that it is desirable to recall the evidence which exists 

 as to the occurrence of chromatic cells in other groups of 

 the Mollusca. 



In Pkyllirhoe remarkable cells containing a brilliant 

 golden-yellow substance were described in 1854 by H. 

 Miiller and Gegenbaur (8). They are grouped in a pair 

 of longitudinal bands, one along the dorsal, and the other 

 along the ventral edge of the body. They are sub-epithelial 

 in position, and their nucleus is easily recognisable in young 

 individuals. They were found to exhibit two modifications 

 of form ; one of these so closely resembles that of the 

 chromatophores of Cephalopods that, although no pheno- 

 mena of contraction or expansion were observed in them, 

 they were interpreted by Miiller and Gegenbaur as of the 

 nature of chromatophores. These curious bodies would 

 well repay a renewed examination. According" to Panceri 

 (9) they have nothing to do with the phosphorescence 

 exhibited by Pkyllirhoe. 



In Pteropods chromatophores were first observed by 

 Kolliker, Miiller, and Gegenbaur (10), and were fully 

 described and figured by the latter naturalist in 1855 

 (11). Two kinds of chromatophores ' were recognised. 

 The first type consists of a central pigment-cell surrounded 

 by a close-set series of radial muscle-fibres, each of which pos- 

 sesses a nucleus and has thus the value of a cell. The whole 

 structure is contained within a well-defined space bounded 

 by parenchymatous connective tissue, which forms a wall 

 for the attachment of the radial fibres. The central pig- 

 ment-cell is expanded by the contraction of the radial fibres 

 as in Cephalopods. This type of chromatophore was found 

 in the fins of several species of Tiedemannia and of Cym- 



