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Show 72 ON CLASSIFICATION. So rne part or other of the integument o.fd all M. ammals exhibits 1 . "hairs "-horny modifications of the ep1 erm1s-": nc1 1. so far resemble feathers, that they are develop:d upon pap1llm Inclosed WI' th1' n sacs,· but, on the other hand, d1ffer from the horny ap-pendages of birds, in n~t . splitting up as they aro protruded, in the fashion so charactenst1c of. feathers. Fig. 36. Fig. 36.-The skull of a Dog.-D. Ramus of the lower jaw; Sq. 'quamosal. Finally, all Mammals are provided with organs for the secretion of a fluid which sub erves the nourishment of the young after birth. The fluid is milk ; the organs are the so-called "mammary" glandsJ and may probably be regarded as an extreme modification of the cutaneous sebaceous glands. These glands are aggregated into two or more masses, disposed upon each side of the median line of the ventral surface of the body; and, in almost all Mammals, the aggregated ducts of each mass open upon an elevation of the skin common to all-the nipple or teat. To this the mouth of the newly-born Mammal is applied, and from it, either by suction on the part of the young, or by the compressive action of a special muscle on the part of the parent, the nutritive fluid makes its way into the stomach of the former. :r. 73 LECTURE V. ON 'fHE CLASSIFICATION OF ANIMALS. ON THE ARRANGEMENT OF THE CLASSES INTO LARGEH. GROUPS. HAVING now arrived at the end of the li~ of classes, and having endeavoured to furnish you with a statement of the structural features cmnmon to, and characteristic of, each class, it will be my next object to discuss the relations of these classes one to another, and to inquire how far they present such common characters as will enable us to group them into larger divisions. And, to commence with the highest classes, it is clear that the Marnmalia, Aves, and Reptilia are united together by certain very striking features of their development. All possess an amnion and an allantois, and are devoid, throughout life, of any apparatus for breathing the air which is dissolved in water. In other words, they constitute what has been termed the "province" of ABRANCHIATE VERTEBRATA, in contradistinction to Pisces and Amphibia, which possess no amnion, nor allantois (or at most a rudimentary one), and, being always provided at a certain period, if not throughout life, with branchim, have been called BRANCHIATE VERTEBRATA. The abranchiate, however, form a far less homogeneous assemblage than the branchiate Vertebrata-Mammals being so strongly separated from Reptiles and Birds that I am disposed to regard them as constituting one of three primary divisions, or provinces, of the Ver·tebrata. The structure of the occipital condyles, the Htructure and mode of articulation of the n1andi- |
