INTRODUCTORY OBSERVATIONS. 85 



the little pisciform creature is gradually assuming the gene- 

 ral characters of the reptile type. I shall here offer a few 

 illustrations of the changes which take place in each of 

 these systems, giving the detail of the various modifications 

 which take place in the different groups, when describing 

 each form. 



The swimming of fishes is effected by the powerful late- 

 ral motions of the tail and posterior part of the trunk, and 

 the numerous short oblique masses of muscle, which consti- 

 tute the greater proportion of the bulk of those parts, are 

 admirably adapted for the performance of these powerful 

 and rapid movements. Such is the use, too, to which the 

 long and powerful tail which belongs to the Tadpole is 

 applied ; and the wriggling motion given to this organ, by 

 which the little animal is propelled through the water, by 

 successive lateral impulses, is similar to that by which 

 fishes are propelled. In the Anoura this is totally re- 

 moved by absorption, and gives place to the extensive and 

 interesting development of the anterior and posterior 

 limbs, particularly of the latter, which are equally adapted 

 for leaping with great force on the land, or for a rapid and 

 equal propulsion through the water. In the Urodela, as 

 the Newts, for instance, this original mode of progression 

 continues throughout life, and the feet which are ultimately 

 added to the animal's organization, are extremely small 

 and feeble, serving only for awkward and imperfect pro- 

 gression on land, or along the bottom of the water. 



The organs of circulation undergo a no less striking and 

 far more important change. In the first instance the heart 

 is single, but the circulation is complete. The blood, after 

 its systemic circulation, is received from the veins by a 

 single auricle, and is immediately propelled by a fleshy 

 ventricle to the branchial arteries, of which one goes to 



