ioi8 



CHAPTER LI. 

 CLASS AMPHLBLA. 



GENERAL STRUCTURE AND ORDERS. 



The Amphibia, which comprise the Frogs and Toads, Salamandroids, 

 Csecilians, and the extinct Labyrinthodonts, and are sometimes 

 known as the Batrachia, agree in so many points of structure with the 

 more generalised Fishes, that Professor Huxley groups the two classes 

 together under the common name of Ichthyopsida. Recent observa- 

 tions on fossil forms, tend, however, to show in the other direction 

 a transition from the Amphibia to the more generalised Reptilia. 

 The Amphibia as a whole differ from Fishes mainly by the circum- 

 stance that when median fins are present they are devoid of fin- 

 rays ; and that the limbs, when present, contain the same skeletal 

 elements as those of the higher classes. They agree with Fishes in 

 having branchiae in their earlier stages of life, but these very fre- 

 quently disappear in the adult, when respiration is carried on entirely 

 by means of the lungs. An epidermal exoskeleton is generally 

 wanting. There is no amnion, and at best but an imperfect allantois 

 in the embryo. In existing forms the cranium always articulates to 

 the vertebral column by two distinct exoccipital condyles, but in a few 

 Labyrinthodonts these were not ossified. The mandible articulates 

 to the cranium without the intervention of a suspensorium ; so that 

 the skull, like that of the Dipnoid Fishes, is autostylic. A large para- 

 sphenoid is always present ; and cranial bones are largely developed, 

 although their number is generally less than in Fishes. The external 

 nares are terminal in nearly all cases. The vertebral column is 

 more or less completely ossified, and can generally be differentiated 

 into cervical, dorso-lumbar, sacral, and caudal regions ; the sacrum 

 but rarely comprising more than a single vertebra. The infraneural 

 segments of the vertebral column are frequently amphicoelous, and 

 in recent forms each bears its own arch ; but in certain Labyrintho- 

 donts, as we shall see below, the neural arches are carried by alter- 



