76 INTRODUCTION TO CLASSIFICATION. 



widely from Birds and Reptiles as the latter are separated from 

 Amphibia and Fishes. 



Thus the classes of the Vertebrata are capable of being 

 grouped into three provinces : (I.) the ICHTHYOPSIDA (com- 

 prising Pisces and Amphibia), defined by the presence of 

 branchiae at some period of existence, the absence of an aranion, 

 the absence, or rudimentary development, of the allantois, and 

 blood-corpuscles which are always nucleated ; (II.) the SAUR- 

 OPSIDA, comprising Reptilia and Aves, defined by the absence 

 of branchiae at all periods of existence, the presence of a well- 

 developed amnion and allantois, a single occipital condyle, a 

 complex mandibular ramus articulated to the skull by a quad- 

 rate bone, and nucleated red blood-corpuscles; and (III.) the 

 MAMMALIA, devoid of branchiae and provided with an amnion and 

 an allantois, but with two occipital condyles and a well-developed 

 basi-occipital ; with a simple mandibular ramus articulated with 

 the squamosal and not with the quadratum; with mammary 

 glands ; with red non-nucleated blood-corpuscles ; and with a 

 corpus callosum in the brain. 



These five classes, whether divided into two or three pro- 

 vinces, again, present so many characters, already enumerated, 

 by which they resemble one another, and differ from all other 

 animals, that, by universal consent, they are admitted to form 

 the group of VERTEBRATA, which takes its place as one of the 

 primaiy divisions or " sub-kingdoms " of the Animal Kingdom. 



The next four classes Insecia, Myriapoda, Arachnida, Crus- 

 tacea without doubt also present so many characters in common 

 as to form a very natural assemblage. All are provided with 

 articulated limbs attached to a segumented body-skeleton the 

 latter, like the skeleton of the limbs, being an " exoskeleton," 

 or a hardening of that layer which corresponds with the outer 

 part of the epidermis of Vertebrates. In all, at any rate in the 

 embryonic condition, the nervous system is composed of a double 

 chain of ganglia, united by longitudinal commissures, and the 

 gullet passes between two of these commissures. No one of the 

 members of these four classes is known to possess vibratile cilia. 

 The great majority of these animals have a distinct heart, pro- 



