THE CHROMATOPHORES OF ANIMALS. 123 



around the pigment-cell which is the active agent in the 

 contraction of the chromatophore. These cells, which cor- 

 respond to the basilar cells of Girod (15), may be the homo- 

 logues of the peripheral cells in Aplysia, while the radially 

 elongated cells may be of more recent origin. 



Whatever may be thought of this suggestion, it certainly 

 seems to me significant that complex chromatophores of the 

 Cephalopod type are also met with in certain Gymnosom- 

 atous Pteropods, as has been described above, — a group 

 of animals which Pelseneer has conclusively shown to be 

 closely related to the Aplysioid Opisthobranchs. 



The Chromatophores of Vertebrata. The chromato- 

 phores of fishes have recently been studied from the physio- 

 logical point of view by Cunningham and MacMunn (21), and 

 from the histological by Ballowitz (22, 23). It is of course 

 with the morphology of chromatophores that we are especially 

 concerned in the present article ; but some of the observa- 

 tions made by Cunningham and MacMunn bear upon one 

 of the points which have been raised. In the pipe-fish 

 Siphonostoma, the chromatophores are stated to lie so deep 

 as to form a black sheet over the dorsal surface of the 

 peritoneum ; while in the flounder, Pleuronectes jlestis, 

 and to a less extent in the dab, P. limanda, pigment exists 

 in the form of irregular branched masses between the epi- 

 dermal cells, often resembling the dermal chromatophores 

 in form, "as though some of the latter had bodily migrated 

 into the ectoderm ". 



In the eel Way mouth Reid (24) has recognised the 

 cellular nature of the branched pigment masses in the ecto- 

 derm, and is strongly disposed to regard them as intrusive 

 mesodermal cells. This is the view which Kolliker (25) 

 was the first to propound as a result of his observations on 

 Lepidosii'cn. In this form numerous pigmented ramifica- 

 tions are found in the epidermis, but the bodies of the cells 

 from which they arise are situated in the superficial layer of 

 the cutis. In the sturgeon, H. M tiller found entire pig- 

 ment-cells within the epidermis, and regarded them as 

 branched ectodermal cells ; but Kolliker suggested that 

 they had intruded from the cutis, Lepidosiren presenting a 



