368 SUMMARY OF CURRENT RESEARCHES RELATING TO 



groups ; in the first all tbe examples have the same colour, whatever 

 that may be ; in the second we have those with bright lateral bands, 

 and a broad dark median band ; in the third there are bright lateral 

 bands, but there is a delicate white median one, and between it and 

 each edge there is a broader dark longitudinal baud ; in the fourth 

 group the examples are sj^otted, and in the fifth there is a kind of 

 transverse striation. In some rare cases individuals were observed 

 which could not be brought into any one of the above categories. The 

 separate groups are fully described. 



In the second or anatomical portion an attempt is made to bring 

 these colours into relation with histological characters ; it has beeu 

 found that all green or greenish colours are not due to the animal 

 itself, but to lower algfe ; similarly diatoms infiuence the coloration 

 in the direction of yellowish-brown, but other animals, such as the 

 Infusoria under the carapace, do not appear to afiect the aj)pearance 

 of the animal. The reddish or greenish-grey colours seen along the 

 median line are due, not to the tissues of the animal, but to the 

 vegetable remains in their intestine. The unpigmeuted or non- 

 coloured spots are referable to the histological elements ; if pale 

 yellow or reddish they have been affected by the chitin ; oil-drops in 

 the hypodermis give rise to a yellow coloration. White, red, and 

 brown bands are to be referred to the chromatophores in the hypo- 

 dermis ; these consist of an upper layer of chitinogeuous cells and of 

 a lower layer formed of a granular protoplasmic mass with nuclei 

 regularly distributed, but without cell-walls. The cliromatophores, 

 which are regularly arranged, have a diameter of from 60 to 80 ^u. 

 They clearly belong to the series of ama^boid cells, and though 

 two kinds of them can be distinguished they are histologically 

 equivalent to one another. 



In the third, or physiological, section the author discusses the 

 influence of the constituents of the environment ; he finds that food 

 has no influence on the coloration, while temperature is frequently 

 seen to be of importance. Light, of course, is still more a factor, 

 while the proportion of salines in the water often greatly aflects 

 the form, size, and coloration. As to the coloration itself, we may 

 see that there is no proof of any warning or protective aim, nor 

 does sexual selection seem to have been of any influence. All the 

 colours and markings of Idotea must be regarded as being sympathetic 

 and referable to adaptations to environment. The colours are found 

 to change rapidly, stages from bright yellowish-brown to dark brown 

 succeeding one another in one and the same animal ; or there may be 

 a direct passage from a light to a dark shade ; brightly coloured 

 specimens, placed in dark vessels, gradually dilate their chromato- 

 phores, while dark examples placed in white porcelain vessels contract 

 their colouring cells. 



The white chromatophores change less rapidly, and moreover 

 function in an opposite sense to the dark ones, for they dilate when 

 the animals become lighter, and contract as they become darker. The 

 change in colour appears to be associated with the presence and 

 functional activity of the optic organs. 



