10 ANATOMY OF THE RABBIT 



tubular central nervous system; a notochord, a fundamental axial 

 support which is only embryonic except in the lowest chordates; 

 and a series of gill clefts, such as occurs in every chordate either 

 in the adult condition, or in the embryo alone. 



These facts may be set down in tabular form, as indicated below. 

 A similar plan can be constructed for any group of organisms, but 

 whether it constitutes a natural or an artificial classification depends 

 on whether or not it is based upon an actual study of the affinities 

 of the organisms concerned. A natural classification should show 

 at a glance not only what the relative importance of any particular 

 character may be, but also how it stands in the scale of specializa- 

 tion. For example, the placental stage of vertebrate development,^ 

 i.e., the development of the placenta itself in the highest stage of 

 vertebrate evolution, is the culmination of a series of arrangements 

 for the care of eggs and young, and the adherence of human 

 structure in the vast majority of features to the type of higher 

 mammals is expressed by the fact that man is also a placental 

 mammal. 



Phylum CHORDATA. Animals with notochord and gills. 



fPROTOZOA, Annulata, Mollusca, etc., invertebrate phyla. 



Subphylum GRANIATA. (VERTEBRATA) Chordates with organized head 



region. 

 fAcRANiA. Lancelets. Also Ascidians and worm-like Chordates, some- 

 times separately classified. 



Class MAMMALIA. Warm-blooded craniates, with hair coat. 

 Young nourished from mammary glands. 



fCvCLOSTOMATA, PiSCES, AMPHIBIA, RePTILIA, AvES, 



lower vertebrate classes. 

 Subclass THERIA. Viviparous mammals. 



IPrototheria. Oviparous mammals. 

 Infraclass EUTHERIA. Placental mammals. 



fMETATHERiA. Viviparous mammals without placenta. 



Order LAGOMORPHA. Gnawing placentals, with chisel-like incisors, of which 

 there is in the upper jaw a small second pair directly behind the 

 main pair. 



fRoDENTiA, Carnivora, Perissodactyla, Primates, etc. Various 

 placental orders otherwise adapted. 

 Family LEPORIDAE. Hares and Rabbits. 



fOCHOTONIDAE. Picas. 



^Equivalent groups. 



