THE CHROMATOPHORES OF ANIMALS. m 



the exterior by a narrow neck, and belonging really to the 

 ectodermal layer (Leydig, Arch. f. Naturg., xlii., 1876; 

 Boll, /. c). This type of pigmentation occurs in Pul- 

 monata ; but there is also in this group, just beneath the 

 epithelium, a dense pigmented network, which consists, 

 according to Leydig, of branched mobile pigment-cells. 



11. Echinoderma. In Holothurians intracellular pig- 

 ment may be restricted to a granular deposit in the ecto- 

 dermal epithelium, e.g., in Sticlwpus regalis, or may occur 

 in the form of bead-like deposits along thread-like pro- 

 longations of the basal ends of ectodermal cells (Jourdain, 

 Ann. M21S. Hist. Nat., Marseille, i., 1883), or in the 

 terminal fibres of the cutaneous nerve-plexus. Ludwig 

 also mentions the existence of richly branched pigment- 

 cells (Bronns Thier-Reichs). 



In Asterids the brilliant red and violet pigments are in 

 the form of granules deposited in the cells of the external 

 epithelium (Cuenot, Arch. Zool. Exp. (2), v., bis 1887- 1890). 

 The epidermal pigment-cells of the optic pits are prolonged 

 at their bases into long threads enclosing bead-like deposits 

 of pigment as in Holothurians (Hamann, Beitrage, Asteri- 

 den, Jena, pp. 17-19, 1885). 



In Echinids the pigment is contained in branched con- 

 tractile cells, which lie either in the connective tissue layer, 

 in the space between the epidermis and the muscle layer, or 

 in the epidermis itself. Hamann interprets these cells as 

 wandering cells of mesodermal origin [Beitrage, Echiniden, 

 pp. 22, 30, 43, 45, 53, 85, 1887). 



12. Tunicata. Pigment is almost entirely restricted to 

 the blood-corpuscles and other mesodermal elements, which 

 are often vividly and variously coloured, e.g., in Botry litis. 

 Mesodermal pigmented cells may penetrate into the 

 cellulose test. Ramified pigment-cells have been observed 

 in Salpa bicandata (Gegenbaur, Kolliker u. Miiller, Zeit.f. 

 w. Zool., p. 331, 1853). 



13. Vertebrata. The branched mobile chromatophores 

 of Vertebrates occur both in the epidermis and the dermis, 

 most frequently in the latter. Their structure is identical 

 in the two cases, and on account of their greater frequency 



