74 ON CLASSIFICATION. 



bular rami, the presence of mammary glands, and the non- 

 nucleated red blood-corpuscles appear to separate Mammals as 

 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 Ichthyoids (comprising 

 Fishes and Amphibia), defined by the presence of branchiae 

 at some period of existence, the absence of an amnion, the 

 absence, or rudimentary development, of the allantois, nucle- 

 ated blood-corpuscles, and, as will be seen by and by, a para- 

 sphenoid in the skull ; (II.) the Sauroids, 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 

 quadrate bone, nucleated blood-corpuscles and no parasphe- 

 noid, comprising Reptiles and Birds ; and (III.) the Mammals, 

 devoid of branchiae and with an amnion and an allantois, but with 

 two occipital condyles and a well-developed basi-occipital and no 

 parasphenoid ; a simple mandibular ramus articulated with the 

 squamosal and not with the quadratum, with mammary glands and 

 with red non-nucleated blood-corpuscles.* 



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

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

 by winch 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 

 primary divisions or " sub-kingdoms '' of the Animal Kingdom. 



The next four classes — Insecta, Myriapocla, 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 segmented bodv-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 



* To these may he added the absence of the corpus cullo&um in the brain of 

 Sauroids and its presence in Mammals. See note p. 89. 



