OCR Text |
Show 12 PROE. W. N. PARKER ON THE [Jan. 16, be seen that the young Echidna resembles Ornithorhynchus in possessing:- 1. A thick horny layer covering the snout, as well as horny teeth and a horny caruncle for breaking open the egg-shell. 2. Glands resembling ordinary sweat-glands opening on the snout as well as on the anterior part of the palate. 3. A highly developed Jacobson's organ, resembling that of Lizards and Snakes, enclosed in an independent tubular cartilage, and possessing a large " turbinal" supported by a cartilaginous shelf continuous with the investing tube. 4. A complicated " maxillo-turbinal." 5. A communication between the two nasal cavities, as in certain birds. 6. Numerous glands in connection with the nasal chamber and Jacobson's organ, including a specially large " septal gland " and a " Steno's gland." Most of these characteristics are peculiar to Monotremes amongst Mammals. On the other band, Echidna differs from Ornithorhynchus in : - 1. The absence of any rudiments of true teeth in the young of 12 cm. in length and onwards, and the early extreme specialization of the entire mouth-cavity. 2. The possession of a mammary pouch in the young female. 3. The less solid character of the nasal capsule, the much higher development of the ethmoid turbinals, and the absence of transverse connective-tissue septa in the anterior part of the nasal cavity. 4. The slightly smaller relative size of Jacobson's organ and of its turbinal, the organ also not extending anteriorly to the nasopalatine canal. Since his communication already referred to, Dr. Symington ' has shown that the Jacobson's organ of Marsupials conforms to the general type met with in the Eutheria, and thus differs markedly from that of the Prototheria. The former may very probably have arisen from the latter, but it has undergone various degrees of degeneration : a very slight step in this direction has possibly occurred in Echidna. Symington is probably correct in his opinion that Jacobson's organ reaches its highest development in the Monotremes-higher even than in Lizards and Snakes, in which it presents many points in common with that of the Prototheria. It certainly seems probable that " Ornithorhynchus is far the most primitive type " 2 of the two Monotremes : the young Echidna resembles Ornithorhynchus much more than does the adult, and is very highly specialized as regards many characters besides those 1 " On the Organ of Jacobson in the Kangaroo and Rock Wallaby {Macropus qiqanteus and Petrogale penicillata)" Journ. of Anat. and Physiol, vol xxvi 'n. s. vol. vi. 1892. Quite recently Rose has described the Jacobson's organ in embryos of the Wombat and Opossum, and has shown that in the former its duct is situated on the floor of the organ as in Ornithorhynchus, and not at its anterior end • and also that a large mucous gland is connected with its posterior end (Anat' Anz. viii. Jahrgang, 15 Sept. 1893, p. 766). 2 W . K. Parker, loc. cit. p. 36. |