TADPOLE, CIRCULATION IX. 683 



of pegs of the shape represented by h, each having a slit i 

 extending more than half-way down it ; the threads are 

 wound round these two or three times, and then the end 

 is secured by putting it into the slit i. The plate is now 

 ready to be adapted to the stage of the microscope : the 

 square hole upon which the foot must be placed is brought 

 over the aperture in the stage through which the light 

 passes to the object-glass, so that the web may be strongly 

 illuminated by the mirror. 



The circulation of the Tadpole is best seen by placing 

 the creature on its back, when w r e immediately observe 

 the beating heart, a bulbous-looking cavity, formed of the 

 most delicate, transparent tissues, through which are seen 

 the globules of the blood, perpetually, but alternately, en- 

 tering by one orifice and leaving it by another. The heart 

 is enclosed within an envelope or pericardium ; and this 

 is, perhaps, the most delicate and certainly the most 

 elegant thing in the creature's organism. Its extreme 

 fineness makes it often elude the eye under the single 

 microscope, but under the binocular its form is distinctly 

 revealed. Passing along the course of the great blood- 

 vessels to the right and left of the heart, the eye is 

 arrested by a large oval body, of a more complicated 

 structure. This is the inner gill, which, in the tadpole, is 

 formed of delicate, transparent tissue, traversed by arteries, 

 and presenting a crimson network of blood-vessels. 



The Tadpole is hatched with respiratory and circulatory 

 organs resembling those of the fish. It lives in, and 

 breathes oxygen from the air contained in, the water, 

 and during the early period of its existence respires ex- 

 clusively by gills. 



It will be remembered that in nearly all fish the heart 

 has but two cavities, an auricle and ventricle ; that the 

 blood of the latter is returned by the veins to the auricle, 

 passes into the ventricle, is then transmitted to the gills, 

 where, being exposed to the air contained in the water, it 

 becomes deprived of carbonic acid, aerated, and rendered 

 fit for recirculation through the system. In the reptile we 

 find a modification of plan. The heart has three cavities, 

 two auricles and one ventricle ; by this contrivance there 

 is a perpetual mixture in the heart of the impure car- 



