THE CHROMATOPHORES OF ANIMALS. 107 



markings of the oral disc in Sagartia troglodytes to the 

 distribution of the various coloured ectodermal eland-cells 

 (Sitztmgsberickte Akad. Wiss. Wien, lxxv., p. 394, 1877). 

 The brown colour of the column and base of Sagartia 

 parasitica is caused, according to the Hertwigs, by the 

 pigmentation of " connective tissue cells ' : within the 

 mesoglcea [Die Actinien, p. 45, 1879). 



The highest development of the pigment-cell in Ccelen- 

 tera is found in the Ctenophora, and in this group the pig- 

 ment-cells are exclusively ectodermal. Apart from the 

 "glance-cells" and fluorescent cells of Cestus Veneris, the 

 former of which seem to foreshadow the iridocytes of 

 higher forms, rose-coloured pigment-cells are found in 

 Callianira, and beautiful chromatophores, yellow, brownish- 

 red, or rose in colour, deck the external epithelium of 

 various species of Bero'c and of Euchlora rubra. The 

 chromatophores of Beroe are stellate, and contract upon 

 stimulation ; they are developed from simple unbranched 

 cells of the. ectoderm (Chun). 



3. Turbellaria. In Rhabdocoelida the body colours 

 are seated as a rule in the sub-epidermic parenchyme 

 (L. von Graff). The pigments may be either (1) granular 

 (the usual type) and distributed in connective tissue cells, 

 e.g., most Accela, or in fibres of the parenchyme, forming a 

 reticulum ; or (2) in solution, either (a) diffused throughout 

 the protoplasm of the cell, or (b) restricted to one or several 

 intracellular vacuoles, e.g., various species of Vortex. 



When the pigment is ectodermal it may be either (1) 

 granular, (2) in solution and diffused through the cell-body* 

 or (3) in the form of coloured rods, e.g., Convoluta flavi- 

 bacillum. 



Among Dendrocoelida the colour of the body in the 

 suckerless Polyclads (Acotylea) may be largely due to 

 curious granular vesicles of unknown significance lying in 

 the ectoderm (Lang, Die Polycladen, 1884). 



In the sucker-bearing Polyclads (Cotylea) pigment is 

 deposited either in vacuolised cells of the ectodermal epithe- 

 lium or in non-vacuolised cells of the mesodermal paren- 

 chyme. In the former case the pigment is mostly yellow, 



