THE CHROMATOPHORES OF ANIMALS. 121 



to the pedal ganglia and their nerves, and to the visceral 

 aanglia and their nerves, while afferent fibres reach the 

 centre from the optic ganglia. Krukenberg's experiments 

 with various alkaloids and muscle poisons showed the 

 probable existence of peripheral ganglia in the skin, as 

 well as of a central organ in the brain. Joubin is the first 

 and as yet the only investigator who has offered a descrip- 

 tion of the nerves themselves, his results being based on 

 the employment of methylene blue. The chromatophoric 

 nerves of the mantle arise from the stellate (pallial) ganglia, 

 and each branch terminates in a chromatophore. The 

 terminal nerve-fibre, according to Joubin, is slightly swollen 

 at its extremity and abuts directly upon the edge of the 

 pigment-cell between two of the radial fibre-cells. This is 

 the only point in Joubin's description which is not very 

 satisfactorily demonstrated, and it is to be hoped that a 

 point of so much importance may soon be examined again. 



The most interesting part of Joubin's paper, however, 

 is his description of the development of the chromatophore 

 in the embryo of Argonanta, in which form the development 

 is apparently less abbreviated than in other types of Cepha- 

 lopod. Joubin shows that the pigmented cell, though ulti- 

 mately embedded in mesodermal tissue, is originally one of 

 the constituent cells of the embryonic ectodermal epithelium. 

 At an early stage it becomes slightly larger than its neigh- 

 bours, and then sinks beneath the surface of the epithelium 

 at the apex of a pit-like invagination of the ectoderm. It 

 then enlarges greatly, detaches itself from the epithelial pit, 

 and becomes surrounded by mesodermal cells which trans- 

 form themselves into the radial muscle-cells. The ecto- 

 dermal invagination closes up. 



In the Cephalopod, therefore, as in the Ctenophore, the 

 pigment-cells are ectodermal ; but in the Cephalopod, as in 

 the Crustacean or the Nemertine, the pigment-cells separate 

 themselves from the layer in which they have originated, 

 and take up a secondary position within the mesodermal 

 tissues. 



In existing Cephalopods the pigment-cell is a true 

 chromatophore, whose only demonstrable function is that 



