. Comparative anatomy. Anatomy, Comparative. ORNITHORHYNCHUS Fig. 21.—Ornithorhynchus is a repre- Fig. 22.—Opossum, the typical genus of sentative of the most primitive group of Didelphians. (Redrawn after Newman.) mammals, the Monotremes. As an egg-laying mammal it bridges over the gulf separating reptiles and mammals. Sub-Class Marsupials The marsupials or didelphia give birth to their young in a most imma- ture state and nourish them for some time in an external marsupial pouch situated on the ventral side of the body of the female. The brain has no corpus callosum. A loose allantoic placen
. Comparative anatomy. Anatomy, Comparative. ORNITHORHYNCHUS Fig. 21.—Ornithorhynchus is a repre- Fig. 22.—Opossum, the typical genus of sentative of the most primitive group of Didelphians. (Redrawn after Newman.) mammals, the Monotremes. As an egg-laying mammal it bridges over the gulf separating reptiles and mammals. Sub-Class Marsupials The marsupials or didelphia give birth to their young in a most imma- ture state and nourish them for some time in an external marsupial pouch situated on the ventral side of the body of the female. The brain has no corpus callosum. A loose allantoic placenta occurs in some. Dasyurus has a yolk-sac placenta. Opossum and kangaroo are well-known examples. All the indigenous mammals of Australia are non-placental. Sub-Class Placentalia The placentals or monodelphia have a placenta, a corpus callosum in the brain, and no marsupial bones. Urogenital and digestive outlets are separated. Placentals are subdivided into at least ten living orders. Order I. Insectivora. The insectivores include shrews, moles, and hedgehogs. They are flat-footed and five-toed, and their denti- tion is also unspecialized, so that they are apparently nearest of surviving forms to the original placental. Order 2. Xenarthra. The xenarthra include part of the group formerly included in the edentates such as the armadillos, sloths and. TUPAIA-AN INSECTIVORE. Fig. 23.—Tupaia, the tree- shrew, an Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Neal, Herbert V. (Herbert Vincent), 1869-1940; Rand, Herbert W. (Herbert Wilbur), 1872-1960. Philadelphia : P. Blakiston's Son
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