CIRCULATION IN TADPOLK 1059 



The external gills reach their highest development on the fourth or 

 fifth day after emersion ; and they then wither so rapidly (whilst 

 being at the same time drawn in by the growth of the animal) that 

 by the end of the first week only a remnant of the right gill can be 

 seen under the edge of the operculum (-2, c), though the left gill 

 (b) is somewhat later in its disappearance. Concurrently with this 

 change the internal gills are undergoing rapid development ; and 

 the beautiful arrangement of their vascular tufts, which originate 

 from the roots of the arteries of the external gills, as seen at g, 5, is 

 shown in 4. It is requisite that the tadpole subjected to obser- 

 vation should not be so far advanced as to have lost its early trans- 

 parence of skin ; and it is further essential to the tracing out of the 

 course of the abdominal vessels that the creature should have been 

 kept without food for some days, so that the intestine may empty 

 itself. This starving process reduces the quantity of red corpuscles, 

 and thus renders the blood paler ; but this, although it makes the 

 smaller branches less obvious, brings the circulation in the larger 

 trunks into more distinct view. ' Placing the tadpole on his back,' 

 says Mr. Whitney, ' we look, as through a pane of glass, into the 

 chamber of the chest. Before vis is 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, entering by one orifice and leaving it by another. The 

 heart (fig. 792, .3, a) appears to be slung, as it were, between two 

 arms or branches, extending right and left. From these trunks (ft) 

 the main arteries arise. The heart is inclosed within an envelope or 

 pericardium (c), which is, perhaps, the most delicate, and is, certainly, 

 the most elegant structure 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. Then it is 

 seen as a canopy or tent, inclosing the heart, but of such extreme 

 tenuity that its folds are really the means by which its existence is 

 recognised. Passing along the course of the great vessels to the 

 right and left of the heart, the eye is arrested by a large oval body 

 (d) of a more complicated structure and dazzling appearance. This 

 is the internal gill, which in the tadpole is a cavity formed of most 

 delicate transparent tissue, traversed by certain arteries, and lined 

 by a crimson network of blood-vessels, the interlacing of which, with 

 their rapid currents and dancing globules, forms one of the most 

 beautiful and dazzling exhibitions of vascularity.' Of the three 

 arterial trunks which arise on each side from the truncus arteriosus, 

 b, the first, or cephalic, e, is distributed entirely to the head, running 

 first along the upper edge of the gill, and giving off a branch, f, to 

 the thick fringed lip which surrounds the mouth ; after which it 

 suddenly curves upwards and backwards, so as to reach the upper 

 surface of the head, where it dips between the eye and the brain. 

 The second main trunk, A, seems to be chiefly distributed to the gill, 

 although it freely communicates by a network of vessels both with 

 the first or cephalic and with the third or abdominal trunk. The 

 latter also enters the gill and gives off branches ; but it continues 

 its course as a large trunk, bending downwards and curving towards 



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