I 



1883.] TONGT3ES OF THE MARSUPIALS. 621 



Thus in I., the lateral organ is certainly primitive, the circum- 

 vallate papillae come nearest to those ot Ornithorhynchus, and the 

 scattered hair-like papillae perhaps show an approximation to the 

 same animal, in which all the back part of the tongue is thickly 

 covered with papillae of this description ; and so also with divisions 

 II. and III. Even the fact that Didelphys, following a different 

 development in another area, should combine some of the characters 

 of two divisions, is exactly what might be expected. The fact that 

 Didelphys retains a distinctly marsupial tongue is a proof of the 

 great persistence in this organ of characters which at first sight 

 appear to be transient, and merely related to food and habits. 



In a paper on " The Tongue of Ornithorhynchus " in the 

 'Quarterly Journal' for July 1883, I suggested that we found in 

 this animal a structure intermediate between the circumvallate 

 papillae and the lateral organ. In this I was wrong ; it is only 

 related to the former, and the latter develops independently in 

 Marsupials, with the appearance of bulbs in the walls of a row of 

 lateral gland- ducts. But my argument that such a structure might 

 represent an ancestral form of a circumvallate papilla — then based 

 on few data — can now be supported by a long series of intermediate 

 forms. 



Looking at this latter question in the light of the observations 

 recorded, the evolution of the- circumvallate papillae and their taste- 

 bulbs is as follows : — Subepithelial tactile end-organs were at first 

 the only means by which food could cause a nervous stimulus. 

 These became more sensitive by the upward growth of the papillary 

 processes (in which they were contained), so that the end-organs 

 were separated from the stimulus by a lessening thickness of 

 epithelium. At the same time sapid substances gained a greater 

 power of penetration by the development of serous glands out of 

 those of the wide-spread mucous type. Probably the gland-ducts 

 surrounded a circular or oval surface in which the end-organs 

 existed. Finally the upgrowth of the end-organ reached a climax 

 in the perforation of the epithelium. At the same time the end- 

 organ must have become gradually modified in a gustatory direction, 

 losing its tactile functions. But at the perforation of the epithelium, 

 the delicate subepithelial end-organ became exposed to the various 

 agencies at work upon the surface of the epithelium. Hence the 

 folding down of the sides of the area, and the opening of ducts into 

 the furrow thus formed, and the protection of nearly all the end-organs 

 {Ornithorhynchus, fig. xii. Plate LIV.). In an exposed part of the 

 tongue of the same animal the protective change was carried much 

 further (fig. xiii. Plate LIV.). Then comes a great gap, during which 

 the papillary subepithelial end-organs disappear (due to their delicacy 

 and their need of protection to such an extent as to cause slight 

 usefulness), and new end-organs are developed from the epithelium 

 of the interpapillary processes. These new terminal organs (taste- 

 bulbs) are met with in Marsupials, with distinct indications of their 

 interpapillary origin. Being thus comparatively recent, traces are 

 retained of the old protection necessary for a more delicate end- 



Proc. Zool, Soc— 1883, No. XLL 41 



