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 m 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 Anowra 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 
