338 MANUAL OF ZOOLOGY. 



By Professor Owen the Vertebrata are divided into the two 

 primary sections of the Hcematocrya and the H&matotherma, the 

 characters of the blood being taken as the distinctive character. 

 The Hcematocrya or Cold-blooded Vertebrates comprise the 

 Fishes, Amphibia, and Reptiles, and are characterised by their 

 cold blood, and imperfect circulation. The Hcematotherma or 

 Warm-blooded Vertebrates comprise the Birds and the Mam- 

 mals, and are characterised by their hot blood, four-chambered 

 heart, and complete separation of the pulmonary and systemic 

 circulations. The chief objection to this division lies in the 

 separation which is effected between the Reptiles and the 

 Birds, two classes which are certainly very nearly allied to one 

 another. 



By Professor Huxley the Vertebrata are divided into the fol- 

 lowing three primary sections : 



I. ICHTHYOPSIDA. This section comprises the Fishes and 

 the Amphibians, and is characterised by the presence at some 

 period of life of gills or branchiae, the absence of an amnion, 

 the absence or rudimentary condition of the allantois, and the 

 possession of nucleated red blood-corpuscles. 



II. SAUROPSIDA. This section comprises the Birds and the 

 Reptiles, and is characterised by the constant absence of gills, 

 the possession of an amnion and allantois, the articulation of 

 the skull with the vertebral column by a single occipital con- 

 dyle ; the composition of each ramus of the lower jaw of several 

 pieces, and the articulation of the lower jaw with the skull by 

 the intervention of an " os quadratum;" and, lastly, the posses- 

 sion of nucleated red blood-corpuscles. 



III. MAMMALIA. This section includes the single class of 

 the Mammals, and agrees with the preceding in never possess- 

 ing gills, and in having an amnion and allantois. The Mam- 

 malia, however, differ from the Sauropsida in the fact that the 

 skull articulates with the vertebral column by two occipital 

 condyles ; each ramus of the lower jaw is simple, composed of 



tutes one of the so-called "foetal membranes," and is thrown off at birth. 

 The allantois (fig. 116, C) is an embryonic structure, which is developed 

 out of the middle or " vascular " layer of the germinal membrane. It ap- 

 pears at first as a solid, pear-shaped, cellular mass, arising from the under 

 part of the body of the embryo. In the process of development, the allan- 

 tois increases largely in size, and becomes converted into a vesicle which 

 envelops the embryo in part or wholly. It is abundantly supplied with 

 blood, and is the organ whereby the blood of the foetus is aerated. The 

 part of the allantois which is external to the body of the embryo is cast off 

 at birth; but the portion which is within the body is retained and is con- 

 verted into the urinary bladder. 



