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Show 420 MR. E. W. L. HOLT-STUDIES IN [May 1, other hand, in the Birkelange the so-called head-kidney is more definite in outline, and its histological structure leaves no doubt that it is perfectly functional. In the Ling the proportion of reniform matter is much less, but the head-kidney does not appear to be wholly f unctionless. The Reproductive Organs.-Three specimens examined proved to be females. The ovaries are much like those of the Ling. From their condition it appears probable that the species spawns off Iceland and Faroe late in the summer or in the autumn. The Skeleton.-One of the Faroe specimens has 78 vertebrae, the first complete haemal arch occurring on the 32nd. Lilljeborg gives the total number as 78-79, of which 30-31 belong to the trunk. In other respects the skeleton is much like that of the Ling. Morphological Considerations. It appears that we have before us a form very closely allied to the common species, and in which almost all the distinctive characters are illustrative of specialization along one definite line. To take them seriatim, w e may consider first the relative length of the jaws. The protrusion of the lower jaw is a feature most obviously associated with the feeding-habits of the fish, and the fact that the Northern Gadoids usually exhibit this character might be found, could the matter be satisfactorily investigated, to indicate nothing more than that the food-supply available in the higher latitudes is not adapted to the requirements of bottom-feeders. At the same time the protrusion of the lower jaw in the larvae of all Teleosteans that have been studied may indicate that this condition in the adult has an ancestral significance, such fish as exhibit it being in this respect less specialized than others which do not, though here again the pelagic habit of the said larvae may go far to prove the character to be merely one of adaptation. Thus, like most other scientific facts in the present state of our ignorance, w e may derive therefrom whatever interpretation appears most convenient to preconceived theory. That there is no necessary connection in Gadoids between this character and an abysmal habitat is sufficiently demonstrated by the Macruridse, but the fact remains that such association usually exists in the less specialized members of the group (cf. Merluccius, Mora, & c ). The reduction of the barbel is a character so obviously related to the elongation of the lower jaw that it requires no separate discussion, since it is difficult to see what benefit au "underhung" fish could derive from such an appendage, however well developed. Nevertheless the presence of a well-developed and functional barbel in the closely-allied species with the shorter mandible almost forces us to regard the appendage in M. abyssorum as vestigial, and, if that be admitted, it is not difficult to arrive at a conclusion as to which condition of the jaws is least indicative of specialization. |