OCR Text |
Show 1894.] VARIATION IN FLAT-PISHES. 247 upon examination found to be an expression of the fact that the ordinal positions of the neural spines crossing the centres of the spots of the dorsal row are, if reckoned from behind forwards, almost exactly the same as those of the haemal spines crossing the centres of the ventral spots. The particulars are as follows:-The centre of the most posterior dorsal spot stands almost exactly over the neural spine of the 11th fin-ray, reckoning from behind. The centre of the next spot is upon that of the 26th fin-ray reckoned from behind; that of the next is on the 42nd. Of the ventral spots the centre of the most posterior is on the haemal spine of the 11th fin-ray from behind ; that of the next is on the 24th, and that of the most anterior is on the 40th. It will be seen that the numbers in the two rows closely correspond. Continuing the dorsal series there is a spot on the neural spine of the 52nd fin-ray, another very minute and faint spot over the 63rd. In front of this there are 16 more fin-rays. The whole number of fin-rays in the dorsal fin is 79, and in the anal fin 58. O n detailed examination it may be seen that the spots are not wholly shapeless blotches of colour, but that some of them consist of several irregular zones of colours. Each of these spots is thus a somewhat indefinite ocellar mark. The spots old, vc, and vb have each a minute centre of light colour, which is chiefly due to the presence of a whitish scale in the middle of the spot. This light colour is not altogether confined to the one scale, but spreads a little on to the edges of the adjacent scales. The spot db has two of these small whitish centres. The spot dc alone of the five chief spots has no light centre. Around the centre of each spot are scales of a brown tint bearing specks of very dark pigment. These deeply pigmented scales form a zone about four rows deep in the case of the larger spots. Outside this is an irregular zone of fine pigment-granules giving a neutral tint. Beyond this again there was in the case of spots dd, dc, db, and vc a vague and imperfect band of silvering, forming a border to the proximal limbs of the spots. It is thus seen that the colours of the dorsal and ventral borders have, so far as tbe last three spots are concerned, varied similarly and simultaneously, producing a result that is nearly symmetrical about the horizontal axis. This phenomenon is precisely comparable with the much more common phenomenon of similar and simultaneous variation of the right and left sides of a bilaterally symmetrical body. It is to be remembered that in many fishes, and especially in Flat-fishes, there is an imperfect relation of bilateral symmetry subsisting between the parts dorsal and ventral to the horizontal median plane. This symmetry is generally manifested both in form and colour, and is an indication that at some time these parts have undergone similar variation. The present example illustrates the principle that parts, which in the normal are in |