the Coloration of Crustacenns. 145 



guislies the flood-tide from the ebb, it is not unusual to find a 

 Somewhat \nrge quantity of a floating weed, Ilalidrt/s sili- 

 rjiiosn^ and nearly always there are to be found in this alga 

 cunsiderable numbers of an Isoj)od Crustacean, Idotea marina, 

 "which in the general shape of its body, and especially in 

 colour, bears a deceptive resemblance to the brownish elongate 

 elliptical floats of tiie weed, which have gained the latter its 

 name. This property of chromatic mimicry which is 

 j)Ossessed by Idotea marina seems to be shared by several 

 other species of the genus. In the case of Idotea tricuspi- 

 dota^ a species which is very common in the vicinity of 

 Saint- Vaast, and especially on the tower which serves as a 

 beacon at the Dranguet reefs, it is not difficult to ascertain 

 that the individuals which live in the aeorn-barnacle zone 

 are usually of quite a different colour from those living amidst 

 the mussels which cover the base of the tower, and that the 

 latter likewise generally differ in tint from those which are 

 to be met with among the Ulva. 



P. Mayer has observed these changes of colour in specimens 

 of Idotea tricuspidata in one of the tanks at the Naples Zoolo- 

 gical Station*, and has found that the same animals change 

 colour according to the objects which surround tliem, and that 

 in the space of half an hour. 



If in some way or other the animal is rendered blind, as by 

 extirpating the eyes for example, this curious adaptation does 

 not take place. Carl Matzdorfff, who has studied very 

 thoroughly the colour- variations of Idotea tricuspidata, has 

 shown that these changes are due to chroraatoblasts which 

 possess the power of expansion and contraction. Tiie indi- 

 viduals which live upon alga3 or hydroids are, according to his 

 observations, in the majority of cases less higlily coloured than 

 those which run about on the bottom. Matzdorff even 

 succeeded in causing specimens of Idotea to change colour by 

 the experiment of placing them in differently coloured glasses. 



Perhaps this adaptation to the general colour of the ambient 

 medium is to be regarded as the cause of the coloration 

 observed in that singular blue pelagic species Idotea annulata ; 

 at any rate its commensalism with a Physalia wdiich is like- 

 wise blue would explain this uiimicry |. Several Laimodi- 

 poda, including species of Proto. Protelluj and Caprella^y 



* P. Mayer, Mittheilungen zool. Stat, Neapel, Bd. i. pp. 520, 521. 



t Matzdorff, " Leber die Fiirbung von Idotea tricuspidata "' (Dissert, 

 inaug., Jena, 1882), Jenaische Zeitschrift, L5d. xvi. p. 158. 



I Speuce Bate, Ann. & Mag. Nat. Hist. ser. 4, vol. i., 1868, pp. 443, 

 447, pi. xxi. fig. 1. 



§ Proto pedaia, Flora., Protella phasma, Latr., Caprella (tqtiilibra, 

 Spence Bate and Westwood: Ilaller, Zeits(hr. f. wisti. Zuol. I3d. xxxiii. 

 1879. p. ;J91. 



