AMPHIBIA. 



91 



of groups having more striking distinctive cha- 

 racters, there is not, perhaps, a more interesting 

 and satisfactory instance in the whole range of 

 the animal creation than is afforded us in the 

 class of amphibia : a circumstance which can 

 only be fully appreciated by following out the 

 structure of each system of organs, first as it 

 exists temporarily in the tadpole, and ultimately 

 in its permanentcondition in the perfect animal. 



The class has been variously divided into 

 groups according to the different views of the 

 naturalists by whom they have been arranged. 

 The division adopted by many zoologists of 

 the present day, according to the mere presence 

 or absence of the tail in the perfect state, is 

 not only liable to the objections which belong 

 to all merely dichotomous arrangements, but 

 appears to be far less natural and less consistent 

 with the physiological characters of the groups 

 than that which may be derived from the 

 absence or presence and the duration of the 

 branchisB. Thus the frogs and toads, which 

 in the adult state have not the vestige of a tail, 

 and the salamanders and tritons, which retain 

 that organ through life, all agree in the early 

 possession of branchiae, which are subsequently 

 lost and replaced by true lungs, and in un- 

 dergoing consequently a total change in the 

 medium of their respiration ; whilst the pro- 

 teus and the siren retain their branchiae, with 

 lungs, (rudimentary at least,) and probably 

 throughout life possess synchronously the two- 

 fold function of aquatic and atmospheric re- 

 spiration. The amphiuma and menopoma have 

 not as yet been observed to possess branchiae 

 at any period of their existence, though further 

 observations are necessary to warrant the con- 

 clusion of an absolute non-existence of a meta- 

 morphosis in these genera. 



It appears to me that no one arrangement 

 hitherto given sufficiently distinguishes the 

 different forms ; and I venture to propose the 

 following modifications as more consistent with 

 the diversities of structure in the different 

 groups. 



Class AMPHIBIA. 



Order 1. AMPHIPNEURTA. 



Body elongate, formed for swimming. Feet 

 either four, or two anterior only. Tail com- 

 pressed, persistent. Respiration aquatic by 

 means of branchiae, throughout life, co-existing 

 with rudimentary lungs. Branchiae external, 

 persistent. Eyes with palpebrae. 



Genera, Proteus, Siredon, Menobranchus, 

 Siren, Pseudobranchus. 



Order 2. ANOURA. 



Body short and broad. Feet during the tad- 

 pole state wanting ; afterwards four, the hinder 

 ones long and formed for leaping. Tail before 

 the metamorphosis, long, compressed ; after- 

 wards totally wanting. Ribs wanting. Ver- 

 tebrae few and anchylosed. Tympanum open. 

 Respiration at first aquatic by branchia? ; after- 

 wards atmospheric by lungs. Branchiae at first 

 external, but withdrawn within the chest before 



the metamorphosis. Impregnation effected ex- 

 ternally during the passage of the ova. 



Genera, Rana, Hyla, Ceratophrys, Bufo, 

 Rliinella, Otilopha, Ductylcthra, Bombinator, 

 Breviceps. 



Order 3. URODELA. 



Body long, slender. Feet always four. Tail 

 long, persistent. Ribs very short. Respi- 

 ration at first aquatic by external branchiae, 

 afterwards atmospheric by cellular lungs. Ver- 

 tebrae numerous and moveable. Tympanum 

 concealed. Impregnation internal. 



Genera, Salamundrina, Salamandra, Molge. 



Order 4. ABRANCHIA. 



Body long, formed for swimming. Feet four. 

 Cranium solid. Tail compressed. Respi- 

 ration by means of lungs only: branchice none. 

 No metamorphosis known. 



Genera, Menopoma, Amphiuma. 



Order 5. APODA. 



Body elongate, slender, anguiform. Feet 

 none. Tail very short, almost wanting. Lungs 

 one larger than the other. (The existence of 

 branchiae at any period of life unknown.) Ribs 

 very short. Sternum wanting. Ears concealed. 

 Impregnation unknown, probably internal. 



Genus, CtKciliu. 



I. Osteology. The changes which take place 

 in the habits and formation of these animals, 

 in their passage from the tadpole or pisciform 

 state to their adult and permanent condition, 

 are not confined to any one system of organs 

 or of functions. The skeleton, the organs of 

 motion, of sensation, and of digestion are not less 

 the subject of these changes than those of 

 respiration and circulation : it will, therefore, 

 be necessary, in treating of each system of 

 organs, to describe not merely their structure 

 in the perfect state, but the less advanced 

 grade of organization from which they emerge 

 in passing from the condition of a fish to that 

 of a reptile. 



In the adult state, however, they are found 

 to vary considerably in the form and composi- 

 tion of the skeleton, according to their habits, 

 and to the existence or absence of a tail. The 

 principle of compensation, or, in other words, 

 the extreme developement of one set of organs 

 at the expense of another, which is so often 

 seen to take place in every form of animals, 

 is here strikingly illustrated. In the frogs, 

 whose movements on land, from their feeding 

 chiefly on terrestrial prey, are necessarily ex- 

 tensive, we find the hinder legs developed to 

 an extraordinary degree, for the purpose of 

 enabling them to take enormous leaps, by 

 which they not only seek or pursue their prey 

 at a distance from the water, but rapidly 

 escape from danger, and rapidly regain their 

 place of refuge in the nearest pond or rivulet. 

 As it is evident that a long tail and a generally 

 elongated body, with a flexible spine, would 

 be not only useless but inconsistent witli these 

 habits, we find these animals absolutely tail- 



