OCR Text |
Show 1894.] TELEOSTEAN MORPHOLOGY. 443 proximal parts of the two oblique muscles, which are, as usual, attached to the posterior face of the expanded front part of the left ectethmoid; these muscles are seen to pass to the eye to which they belong by dipping under the pseudomesial process. O n turning the specimen over and laying bare the left side of the skull, it is found to have the appearance shown in Plate X X X . fig. 9 ; a figure of the skull of a normal Sole (fig. 10) is added for purposes of comparison. Below the pseudomesial is seen in either specimen a tolerably large foramen, which is considerably largest, however, in the abnormal fish. It would appear from Mr. Cunningham's figure (op. cit. pi. xi. fig. 6) that it m a y be much smaller in normal Soles than in the specimen from which m y drawing was made, but the bones of Teleosteans are notoriously variable. This foramen in normal Flat-fish, as w e have seen in the second part of this paper, puts the left accessory visual organ into connection with the left orbital cavity and gives exit to a cranial nerve. In the specimen before us, however, the whole of the left orbital apparatus protrudes through this foramen, the left accessory visual organ being rather backwardly displaced. It was not very well preserved, and I did not ascertain the position of its opening into the orbital cavity, but this appeared to be external to the skull. The eye itself is resting internally against the left ectethmoid and the anterior part of the pseudomesial. The length of the oblique muscles does not allow it any outward displacement, and the width of the foramen is not sufficient to allow it to be drawn inwards. The recti muscles pass to their usual place of insertion on the inner face of the parasphenoid, and the optic nerve to its origin on the ventral face of the brain. Comparing the normal and abnormal skulls from the left side we find only two points of difference, viz. (a) the foramen is largest in the abnormal example, (b) the left ectethmoid is less developed and lacks the usual antero-dorsal prominence in the abnormal example. That the left ectethmoid has nevertheless undergone the usual rotation is shown by the position of the insertions of the oblique muscles. W e have already noticed from the outside that the eye is partially withdrawn from the surface. Internally its connection with the skin is very slight, in fact it is difficult to avoid separating the outer from the inner layer of the cornea in making the necessary observations. The mandibulary branch of the V-cranial, passing directly below the eye-muscles, appears in some degree to restrict the eye to the position it n o w occupies. To recapitulate, w e have before us a specimen in which there has been practically no migration of the left eye, and yet in which the skull presents, save for a slight deficiency in the left ectethmoid, all the characters which one finds in a specimen in which the usual migration has taken place. The rotation of the mouth-apparatus is quite normal, but I have omitted to consider this because, as Cunningham truly remarks, it is a feature entirely independent of that of the orbital part of the skull. |