ii2 SCIENCE PROGRESS. 



in the mesodermal tissues and their resemblance in form to 

 various unpigmented cells of the connective tissues the 

 chromatophores both of the epidermal and subjacent layers 

 are usually regarded as being of mesodermal origin. 



Pigment is also frequently deposited in the ordinary 

 unbranched cells of the deeper layers of the epidermis 

 (Kolliker, Handbuch der Gewebelehre, Leipzig, 1889). 



From the above survey it may be seen that although 

 pigment-cells are widely distributed, they attain a high 

 elaboration of form and function in relatively few phyla of 

 the animal kingdom. In its highest form the pigment-cell 

 is mobile and under the influence of the nervous system ; 

 but such chromatophores are found only among Vertebrata, 

 among Cephalopod and Pteropod Mollusca, among Crust- 

 acea, and possibly in a few pelagic Annelida and Ctenophora. 

 Branched pigment-cells are also found in Porifera, Nemertea,. 

 in the compound eyes of various Tracheata, in Pulmonate 

 Mollusca, and in Echinoderms ; but in these cases the pig- 

 ment-cell is apparently incapable of those movements which 

 in the chromatophore of the Cephalopod or Vertebrate- 

 cause marked alterations in the colour of the animal. It is 

 even questionable whether the branched form of these pig- 

 ment-cells is in any way connected with purposes of colour 

 display. It is impossible to maintain that colour in itself 

 is of any value to a sponge; and the pigment-cells of Echinids 

 have clearly not been specialised for chromatic purposes,, 

 since the majority of them are situated within the shell 

 where they cannot affect the coloration of the animal. 

 Moreover, in Holothurians pigment-laden wandering cells 

 have been shown to discharge an excretory function (Schultz, 

 Biol. Centr., xv., p. 390, 1895). The function of the pig- 

 ment-cells in Nemertea and Pulmonata is quite problematic; 

 but a nutritive or excretory function could be assigned to 

 them with just as much, or little, reason as could a chro- 

 matic function. 



If we now take into consideration the immobile un- 

 branched type of pigment-cell as found among the Tur- 

 bellaria or the Oligochaeta sEolosoma and Ctenodrihis, we 

 again find that the modifications which these cells exhibit 



