Classification of mammals

The essential characteristic distinguishing mammals from other animal species is the ability of the female of the species to produce milk in specialized organs (mammary glands) for the nutrition of its newborn. 

The class Mammalia is divided into three subclasses  

Eutherians. About 95% of all mammals belong to this subclass. The developing embryo in utero receives nourishment via the placental blood supply (they are referred to as placental mammals) and is born at a high, but variable, species-related state of maturity. All eutherians secrete milk, which, depending on the species, is more or less essential for the development of the young the young of some species are born sufficiently mature to survive and develop without milk. 

The number and location of mammary glands varies with species from two, e.g. human, goat and sheep, to 14-16 for the pig. Each gland is anatomically and physiologically separate and is emptied via a teat. 

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We have many times emphasized that all life has very similar chemistry. Indeed, in terms of biochemistry there is little need for the classifications of mammals, plants, and so on. There is only one important division—into prokaryotes and eukaryotes. Prokaryotes, which include bacteria, evolved first and have simple cells with no nucleus. Eukaryotes, which include plants, mammals, and all... 

McKenna M.C. and Bell S.K. (1997). Classification of mammals above the species level. Columbia Univ. Press, New York, NY, USA. 

Cope (1883) continued the use of grade taxa in his classification of mammals in which groups were erected specifically to reflect some estimate of evolutionary level (for details see Polly, 1993). 

Simpson, G.G. (1945) The principles of classification and a classification of mammals. Bulletin of the American Museum of Natural History, 85 1-350. 

Ceramidases are enzymes that cleave the N-acyl linkage of Cer into SPH and free fatty acid. They are an emerging class of enzymes composed of multiple isoforms. Historically, these isoforms have been classified as acid, neutral or alkaline, based on the pH optimum of their activities although some isoforms show activity in a broad range. With the recent cloning of several isoforms from yeast, bacteria, and mammals, a genetical distinction and classification of these enzymes can now be employed. 

Most marine biotoxications are caused by fishes, but turtles and mammals have been incriminated in rare instances. A tentative classification of ichthyotoxic fishes is as follows ... 

The inability of mammals to synthesize all of the amino acids they require has led to the classification of amino acids as essential and nonessential. Conceptually this distinction seems clear. Practically speaking, it turns out to be more complex to designate essential amino acids. An essential amino acid, in this classification, means one that must be supplied in the diet if the organism is to maintain a positive nitrogen balance. As we will see, the absence of a de novo pathway for the biosynthesis of an amino acid does... 

Table 1 Classification of collagen in mammals Known and estimated information about collagen...

Fig. 2 Classification of Fish and Mammalian ORs. OR genes can be separated into two classes based on their sequences. Mammals have both Class I and Class II ORs, whereas all fish ORs belong to Class I. Insect ORs are quite different from each other and also different from fish and mammal ORs. All Class I ORs are shaded in blue. Class I ORs from mammals are shaded in orange and fish ORs are shaded in green...

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