HIPPOLYTB VARIANS. 625 



frequently obtained specimens of Mysis (Macromysis 

 neglectus and M. inermis^) of a transpai'ent bluish-green 

 colour, and other Mysidee of a biglily transparent, colourless 

 appearance, wbicli recovered to a more or less pronounced 

 brown colour on exposure to light. Pandalus annuli- 

 cornis, which varies in colour during the day between green 

 and red, exhibits a transparent and colourless (or slightly 

 yellowish) nocturnal aspect. Other species of Hippolyte 

 (H. spinus and H. pusiola), brightly coloured by day, have 

 a faint bluish and very transparent nocturnal coloration. 

 We are not aware that this recurring colour-change at night 

 has been previously described in any Crustacea, but the 

 observations made by Verrill on several species of fish 

 (1897) show that they exhibit, when asleep at night, a colour 

 and position very different from the tints and attitudes 

 which they manifest during the day. 



On bringing a number of nocturnes of Hippolyte 

 varians into the light, or on flashing a lighted magnesium 

 ribbon over a dish containing them, the prawns recoil from 

 the light with a sudden and violent start, and swim actively 

 about. When rapidly examined under the microscope by 

 incandescent gas-light their '^chromatophores '^ undergo 

 such speedy change, that in a minute or less the transparency 

 has gone and the diurnal colour returned. In winter, 

 however, the change is generally much slower, so that it is 

 possible to examine them even with high powers and to 

 discover the source of the blue colour. The transparency of 

 the body is wonderful, and, as Professsor Sars found in 18G7 

 to be the case with Mysis, greatly facilitates the study of 

 their anatomy. The outlines of the individual strands of 

 muscle, the connective-tissue networks and bridles, the 

 muscles of the heart and its ostia, the arteries and their 

 branches, with the blood-corpuscles swiftly flowing along them 

 and returning more leisurely by the veins, the gills, the 

 alimentary canal, the nerve-cord, the optic ganglia, can all 



1 We are indebted to A. O. Walker, Esq., F.L.S., and W. I. Beaumont, 

 Esq., for their kind aid in the identification of these Crustacea. 



