Mammalia/mammals

Giant pandas are descended from the same ancestral carnivores as bears, raccoons, dogs, and cats, and so are placed in the order Carnivora (carnivores) within the class Mammalia (mammals). Pandas have, however, almost entirely lost the meat-eating habit. It is not yet fuUy resolved whether giant pandas are more closely related to the bear family (Ursidae) or the raccoon family (Pro-cyonidae). 

Comparatively little Is known about the species distribution of the various methylation reactions. Catechol and N-metlqrlatlons apparently occur throughout the Mammalia (46) In the case of azaheterocycle pyridine there occurred at 10 fold variation In the extent of methylation (48), from the rabbit and cat which are extensive methylators (up to 50Z of the dose), to rat, mouse and man. In which methylation only accounts for 2-lOZ of an administered dose. It Is well known that the 0-methylatlon of 4-hydroxy-3,5-dllodobenzolc acid Is far more extensive In primate species than non-prlmate mammals (52). 

Classification is difficult since Ig of different classes may still have some sequence homology and, hence, cross-react, or have similar biological functions but different physicochemical properties. For example, IgY of birds (Leslie and Clem, 1972) which in biological respect resembles mammalian IgG, is in physicochemical respects quite different (e.g. contains an extra domain in the heavy chain). Tentative conclusions which may be drawn are (i) homologs of mammalian IgM seem to exist in all vertebrates (ii) IgA is found in mammals and birds, IgE in mammals and, perhaps, birds (iii) IgG is restricted to mammals only and (iv) vertebrates other than mammalia may produce other Ig such as IgY and IgN (Fig. 6.1). 

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