384 MANUAL OF ZOOLOGY. 



hence this section is spoken of as the Amniota or as the 

 Attantoidea* 



By Professor Owen the Vertebrata are divided into the two 

 primary sections of the H&maioerya and the Hceinatotherma, 

 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 charac- 

 terised by their cold blood and imperfect circulation. The 

 Hamatothirma or Warm-blooded Vertebrates comprise the 

 Birds and the Mammals, 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 

 following 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- 



* The amnion (fig. 153) is a membranous sac, containing a fluid the 

 liquor amnii and completely enveloping the embryo. It constitutes 

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

 allantois (fig. 153, C) is an embryonic structure, which is developed out of 

 the middle or "vascular" layer of the germinal membrane. It appears 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 allantois in- 

 creases 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 fcetus 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 converted into 

 the urinary bladder. 



