DIVISIONS OF THE VERTEBRATA. 337 



organs are very rudimentary. Touch, or " tactile sensibility," 

 is usually possessed to a greater or less degree by the entire 

 surface of the body ; but the sense of touch is generally localised 

 in certain particular parts, such as the appendages of the mouth, 

 the lips, the tongue, or the digits. 



In all Vertebrata without exception reproduction is carried 

 on by means of the sexes, and in all the sexes are in different 

 individuals. No Vertebrate animal possesses the power of re- 

 producing itself by fission or gemmation ; and in no case are 

 composite organisms or colonies produced. Most of the Ver- 

 tebrates are oviparous, that is to say, the ova are expelled from 

 the body of the parent either before or very shortly after im- 

 pregnation. In other cases, the eggs are retained within the 

 body of the parent until the young are hatched, and in these 

 cases the animals are said to be ovo-viviparous. In other cases, 

 again, not only is the egg hatched within the parent, but the 

 embryo is retained within the body of the mother until its de- 

 velopment has been carried out to a greater or less extent; and 

 these animals are said to be viviparous. 



DIVISIONS OF THE VERTEBRATA. The sub-kingdom Verte- 

 brata is divided into the five great classes of the Fishes (Pisces), 

 Amphibians (Amphibia), Reptiles (Reptilia\ Birds (Aves), and 

 Mammals (Mammalia). So far there is perfect unanimity; but 

 when it is inquired into what larger sections the Vertebrata may 

 be divided, there is much difference of opinion. Here, the 

 divisions proposed by Professor Huxley will be adopted, but it 

 is necessary that those employed by other writers should be 

 mentioned and explained. 



One of the commonest methods of classifying the Vertebrata 

 is to divide them into the two primary sections of the Branch- 

 iata and Abranchiata. Of these, the Branchiate section in- 

 cludes the Fishes and Amphibians, and is characterised by the 

 fact that the animal is always provided at some period of its 

 life with branchiae or gills. The Abranchiate section includes 

 the Reptiles, Birds, and Mammals, and is characterised by the 

 fact that the animal is never provided at any time of its life 

 with gills. Additional characters of the Branchiate Vertebrates 

 are, that the embryo is not furnished with the structures known 

 as the amnion and allantois. Hence the Branchiate Vertebrates 

 are often spoken of as the Anamniota and as the Anallantoidea. 

 In the Abranchiate Vertebrates, on the other hand, the embryo 

 is always provided with an amnion and allantois, and hence 

 this section is spoken of as the Amniota or as the Allantoidea* 



* The amnion (fig. 116, C) is a membranous sac, containing a fluid 

 the liquor amnii and completely enveloping the embryo. It consti- 

 VOL. II. Y 



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