HIPPOLYTE VARIANS. 601 



cent.) is decidedly the best, and retains the " chromatophores '* 

 and their pigments remarkably well. It is the only reagent 

 which preserves the blue colour of some of these prawns, but 

 the inevitable loss of transparency destroys the delicacy of 

 the azure tint of the living animal. 



For histological work on the chromatophores, owing to the 

 complete solubility of the pigments of the body in alcohol of 

 higher grades and their alteration by other hardening re- 

 agents, we have been compelled to rely very largely upon 

 careful and repeated observation of living transparent speci- 

 mens. Only in this way can a true idea be formed of the 

 several pigments, of their activities towards different stimu- 

 lation, and of the relations of the chromatophores to the 

 vascular, muscular, and nervous systems. For staining 

 chromatophores in the fresh state, methylene blue was given 

 a short but not altogether satisfactory trial. For subsequent 

 observation material was fixed with osmic acid or its vapour, 

 washed with weak chromic acid, and preserved in alcohol. 



Section II. 



Some Colour Forms of Hippolyte varians. 



The close resemblance of Hippolyte in colour and in 

 pattern to its surroundings has been remarked by many 

 naturalists. Not merely are the markings and tints similar 

 to those of the weed or zoophyte amongst which it lives, but, 

 as in many other cases of '' protective resemblance," the 

 animal when discovered remains quiescent, often requiring 

 a vigorous shake to induce it to let go its hold. When dis- 

 placed it jumps towards some other object, on reaching 

 which it attaches itself and instantly subsides again into its 

 quiescent condition. That the prawns exert powers of 

 selection with respect to their weed will be readily realised 

 from Pis. 32 and 33, figs. 1 to 9, representing prawns placed in 

 a dish with sea water, to which subsequently pieces of dif- 

 ferent coloured weeds were added. The prawns were left 



