II. A I i-isiiKs (111- ri:i<i)S(iM \ r \i 



to liave been (leri\e(l Imni a vertically barred pattern, ^ueh as occurs in many I'crcoiils 

 ami in the yonn^ I'siUodfs. " The vertical bars, " he writes, " are not harmful (and 

 may be protective) in a vertically swimming fish with only the vertical shadows to 

 liide in, but this break-up of the surface is essential, except in very special circum- 

 stances, to a fish lying closely on the bottom." Hewer also investigated the normal 

 habitat of the species in question, and found that, in spite of the possession of the 

 pcrmairent pattern of spots ami markings, presumably inherited from ancestral forms, 

 there was a consider, iblr measure of agreement in every case lietween the general 

 effect of the pattern of a particnlar species and the colour antl pattern of its normal 

 environment. 



With regard to the mechanism of the colour changes, Ballowitz (1893) and others 

 have shown that the ehroniatophores in the skin are surrounded by a dense network 

 of nerve-fibres, and the experiments of Pouchet (1876) on the Turbot and other fishes 

 showed conclusively that the responses arc controlled by stimuli received through the 

 eyes by way of the central and sympathetic nervous systems. It had long been 

 known that blind fishes were incapable of adaptive colour changes, and I'ouchet' 

 demonstrated that if the eyes are destroyed, or if the optic nerves or the main sympa- 

 thetic trunks are severed, changes in the chromatophores do not take place. The 

 experiments of Sumner and Mast have confirmed these main conclusions, and the 

 fact that the colour changes in Flatfishes are controlled by the nervous system, and 

 that the necessary stimuli are recei\ed through the eyes, may be regarded as established. 

 Mast further concludes that the ability of the fishes which he studied to adjust the 

 relations between the various types of chromatophores is due to the fact that they 

 posse.ss colour vision. Thus, although it is unlikely that there is any actual visual 

 comparison of the skin with the ground, it is necessary for the fish to survey the 

 ground around it bef<-)re it is able to simulate the colour and pattern. This survey 

 IS rendered possible liy the possession of an accessory organ, the recessus orbitalis 

 (Holt, 1894),'- associated with each eye, peculiar to the Flatfi,shes. This is a .sac-like 

 evagination of the membranous wall of the orbit, which, like the cavity of the orbit 

 itself, IS filled with fluid. By means of this apparatus the eyes can be protruded 

 above the surface of the head to a rem.irkable degree, an obvious advantage to a fish 

 lying more or less buried m the sand Once protruded the eyes can be moved freely 

 and independenth" m .ill directions, <ind rotr.u tion is subsequently effected by means 

 of the eve-muscles 



IV. .ALBINISM, .\MBIC()L()k.ATIO\ .\ND KEVERS.\I..^ 



The percenta.t;c of " sports " is perhaps higher in the Heterosomata th.in m any 

 other grouj) of fishes. Some of these are of great interest and are dealt with in this 

 .section, but there is no evidence at all that they are in any way connected with the 

 problem of the origin of species.'' 



' Similar results lu.vc been obtained ni Ii^Iks ..tljir lli.in Matfishes by Secero\ , Friscli and 

 fither workers. References to their p.ipers will he foun.l in the bibliography of .Masfs paper (toil)). 



- See also Cole and Johnstone (iodj, p. 176). 



^ I take this opportunity of placing on record in\' indebtedness to a number of gentlemen and 

 institutions, who have sent me specimens of abnorinally coloured Flatfishes during the last few 

 years. My thanks are especially due to the Fishmongers' Company, the Marine Biological Associa- 

 tion. Plvmouth. and to Messrs'. .■\. F. Brunner. D. W. Collings, I-''. M. Davis, J. T. Jenkins, A. H. 

 Patterson, F. Smith. I. I,. Stewart, and \V. H. Webster. 



' The literature dealing with albinism, ambicoloration and other abnormalities of coloration is 

 an extensive one. References to many of the works are given in the systematic part under the 

 individual species. The following should be consulted in connection with ambicoloration, etc. : 

 Ritzema Bos (18S-). Cunningham (1801), Giard (r8o2A, i8r)::B), Cunningham (1802, 1893), 

 Cunningham and McMunn (1804), Hateson (1804), Holt (1S.14), Cunningham {1S05, 1007), Franz 

 liomi, (.einiMiU (Piij), Hussaknf (1.114I, S. hnakcnbei k (PiJ;), Xorman (I'l^i.u. v):7^)- 



