GENERAL CHARACTERS OF THE SUBKINGDOMS. 107 



the spleen, is always present in the Yertebrata. The law of bilateral sym- 

 metry prevails in a most perfect manner in the animal organs of locomotion 

 and sensation ; the organs of the senses are also double, excepting in the case 

 of the single olfactory organ of the Amphioxus. Some of the Vertebrata, viz., 

 the Mammalia, Aves, Reptilia, and higher Amphibia, breathe by lungs only ; 

 of the rest, a few of the Amphibia respire by lungs and gills, whilst all of the 

 lowest class, Pisces, breathe by gills alone. Certain distinctive peculiarities, 

 derived from the mode of development of the body of the embryo, and of the 

 so-called visceral arches, belong to this subkingdom ; these will be described in 

 the chapter devoted to the subject of Development. 



The vertebrate animals have been variously subdivided into groups, larger 

 and fewer in number than the classes. Thus, the Mammalia and Birds form 

 the so-called warm-blooded Vertebrata ; whilst the Reptiles, Amphibia, and 

 Fishes constitute the cold-blooded Vertebrata. A more scientific division re- 

 quires, however, the recognition of the intimate connection between the Birds 

 and Reptiles. It has been pointed out that the classes Mammalia, Aves, and 

 Reptilia have, at no period of their existence, gills or organs intended for 

 aquatic respiration ; hence they have been grouped together as the Abranchiate 

 Vertebrata ; whilst the Amphibia and Pisces, always having temporary or 

 permanent gills, have been included under a second group of Branchiate Ver- 

 tebrata : these two groups are also distinguished by certain embryonic charac- 

 ters, which can only be alluded to here, the former possessing the so-called 

 amnion and allantois, whilst the latter is destitute of both those structures, or 

 possesses merely a rudimentary allantois. By another mode of classification 

 (Huxley), the Abranchiate classes of the Vertebrata are separated into two 

 groups, the one containing the Mammalia, and the other the Birds and Rep- 

 tiles. The higher group, Mammals, have no branchiae, but possess an am- 

 nion and allantois : they have two condyles to the occipital bone, a well- 

 developed basi-occipital bone, no parasphenoid bone, and a simple lower jaw 

 articulated with the squamosal and not with the quadrate bone ; they possess 

 mammary glands and non-nucleated colored blood-corpuscles. The lower 

 group, Sauroids, comprising the Birds and Reptiles, likewise have no branchiae, 

 but possess an amnion and allantois : they have no parasphenoid bone, only 

 a single occipital condyle, and a complex lower jaw articulated to the quad- 

 rate bone ; they are unprovided with mammary glands, and have nucleated 

 colored blood-corpuscles. A third distinct group, Ichthyoids, including the 

 Amphibia and Fishes, possess branchiae at some period of their existence, 

 have no amnion, either no allantois or merely a rudimentary one, a parasphe- 

 noid bone in the skull, and nucleated colored blood-corpuscles. 



It would be beyond the scope of this work to define the classes of the animal 

 kingdom ; but the characters of many of their internal organs will be given in 

 the subsequent chapters on the special functions. With regard to the class 

 Mammalia, however, in which man is included, an enumeration of the various 

 orders of animals contained in that class is requisite, since frequent reference 

 is made hereafter to the structure of the digestive and other organs in those 

 groups. As usually defined, these orders are twelve in number, viz., the 

 Bimana, Quadrumana, Carnivora, Cheiroptera, Insectivora, Hodcntia, Ruminan- 

 tia, Pachydermata, Edentata, Cetacea, Marsupialia, and Monotremata. The 

 first of these orders, Bimana, contains the genus Homo, or Man, alone, who 

 is regarded by the highest zoological authorities as of one species, although 

 presenting many varieties. By Linnaeus, however, Man was placed, together 

 with the so-called Quadrumana, the apes, baboons, monkeys, and lemurs, and 

 even the Bats, in a single order named the Primates. The above-named classes 

 are grouped into three divisions ; the first, named the Deciduate-placental 

 Mammalia, includes the Bimana, Quadrumana, Carnivora, Cheiroptera, In- 

 sectivora, and Rodentia ; the second, called the Non-deciduate-placental Mam- 

 malia, comprises the Ruminantia, Pachydermata, Edentata, and Cetacea ; 

 the third, named the Implacental Mammalia, includes the Marsupialia and 

 Monotremata. 



Mollusca. These animals, named from their soft structure (mollis, soft), 

 have no internal segmented skeleton, like the Vertebrata ; nor is the body 

 segmented like that of the Annulosa, to be presently described ; on the con- 



