TADPOLE, CIRCULATION IN. 685 



transparent coil. Through the empty coil the artery is seen 

 on each side descending from the gills, converging to the 

 spine, meeting its fellow, and with it uniting to form the 

 abdominal aorta, the large central vessel coloured red in 

 the figure. After the aorta has supplied the abdominal 

 viscera, a prolongation, or caudal artery is seen descend- 

 ing to the tail, the all-important organ of locomotion in 

 the tadpole. This artery, entering the root of the tail, is 

 imbedded deeply in the flesh, whence it emerges, and then 

 continues its course, closely accompanied by the vein, to 

 within a short distance of the tail's extremity, wliere, 

 being reduced to a state of extreme fineness, it terminates 

 in a capillary loop, which is composed of the end of the 

 artery and the beginning of the vein. The artery, in its 

 course, gives off branches continually to supply the neigh- 

 bouring tissue. We may often observe that the blood- 

 current in the tail, even in the main artery or vein, is 

 sluggish or even still. This occurs independently of 

 the heart, which may continue to beat as usual ; it 

 happens, because the circulation in the tail depends very 

 much on the motion of the organ. When this is sus- 

 pended (as in confining the tadpole under the microscope), 

 the blood moves sluggishly, or stops, till the tail regains 

 its freedom and motion, when the activity of the current 

 is restored. This fact is thus alluded to by Dr. 

 Grant : — " It is the restless activity of the worm and of 

 the insect that makes every fibre of their body, as it were, 

 a heart to propel their blood and circulate their fluids, 

 while the slow-creeping snail that feeds upon the turf has 

 a heart as complicated as that of the red-blooded, verte- 

 brated fish, that bounds with such velocit}'- through the 

 deep. It is because the fish is muscular and active in 

 every point that it requires no more heart than a snail to 

 keep up the necessary movements of its blood." 



Having traced the arterial system, which conveys the 

 blood from the heart to the extremities, we will now note 

 its return bv the veins back again to the heart. 



The caudal vein runs near to the artery during the greater 

 part of its course, with its stream, of course, towards the 

 heart. This stream is swollen by perpetual tributaries from 

 vessels so numerous that their loops form a network which 



