| OCR Text |
Show 624 MR. E. B. POULTON ON THE [Dec. 18, Plate LV.). It now takes the form of a series of slit-like depressions in the smooth epithelium beneath the papillate surface. There are no mound-like elevations, and the whole appearance more resembles that of the well-marked organ of certain higher mammals. Corresponding with this, the sides of the depressions are crowded with closely packed taste-bulbs, and the gland-ducts seem to begin where the taste-bulbs end. Without the knowledge derived from the proceeding species, there would be no suggestion that the gustatory part of the depression is itself a gland-duct (see fig. xxxii. Plate LV.). This well-marked organ is still behind the most complex organ of Eodents in that there is no indication of a lateral area upon which the depressions are arranged, and no elevation of the ridges between the depressions ; in fact the attention is merely directed to the slits, while in the more complex organ the ridges also attract notice. Further the slits are less uniform in size and less regular in arrangement than in the well-marked organs of Eodents, &c. The filiform papillae (/. /. p.) have the usual arrangement; many of them have the shape of a triangular flap attached along the base. The circumvallate papillae (c. v. p.) are arranged in the normal manner; the two anterior papillae are smaller than the posterior and of a different shape (compare figs. xvi. and xx.), the former following the type of Halmaturus and Macropus, the latter resembling the papilla? of higher mammals. The upper surface of the organ is, as usual, covered with coronate papillae. This is also true of the other tongues figured (and probably of all Marsupials). Fig. ix. Natural size. The same tongue (of Phalangista vulpina), seen from the right side. The references are the same as those previously used. Fig. x. Natural size. The tongue of Belideus breviceps, seen from above. There is a lateral organ present (hardly visible from above) in the same situation as that of Phalangista and of similar structure. The relation of the anterior circumvallate papillae (c. v. p.) to the posterior papilla is also similar to that described in Phalangista (compare figs. xv. and xvii.). The anterior contour of the tongue may not be correct. The drawing was made from a spirit specimen in which the tongue was bitten through at the tip, and the anterior narrower part was bent down abruptly. In the figure I have assumed that this was accidental, and this was probably the case. Fig. xi. Natural size. The tongue of Bidelphys quica seen from above. The cavity around the epiglottis (Ep.) has been widened by separating the walls with a needle (drawn in the figure). The normally arranged circumvallate papillae (c. v. p.) appear to resemble one another, and to follow the higher type. It was impossible to be certain of this, because there had been considerable alteration in the spirit specimen. There appears to be no lateral organ. The transverse ridges in front of the circumvallate papillae m a y be accidental. The anterior contour of the tongue m a y not be quite correct. The coronate papillae covering a well-marked patch behind the tip (s. c. p.) are peculiarly modified, a change being very distinct to the naked eye. The posterior part of the ring of secondary papillae is occupied by a single, very strong, cornified, recurved hook (see fig. xxn. Plate LV.). The seven succeeding figures (XII. to XVIII.) illustrate a gradual transition from the circumvallate papilla? of Ornithorhynchus to those met with in the higher mammals. This transition is from a bilaterally symmetrical structure, with taste-bulbs developed over its entire surface, to a radially symmetrical structure with the taste-bulbs confined to a belt round the base of the papilla. At first the papilla is but slightly withdrawn from the surface (fig. xn.); then it is deeply placed at the bottom of a narrow cleft (fig. XIII.) ; it then gradually emerges through a loDg series into the usual type of higher mammals (figs. xiv. to XVIII.). Fig. xn. X14-5. Transverse section of the posterior bulb-bearing ridge of Ornithorhynchus. It is probable that taste-bulbs were first developed |
