CIRCULATION OF BLOOD 1057 



the observation may be overcome by gradually mixing some warm 

 water with that in which it is swimming until it becomes motion- 

 less ; this usually happens w r hen it has been raised to a temperature of 

 between 100 and 110 Fahr. ; and, notwithstanding that the muscles 

 of the body are thrown into a state of spasmodic rigidity by this 

 treatment, the heart continues to pulsate, and the circulation is 

 maintained. 1 The larva of the water-newt, when it can be obtained, 

 furnishes a most beautiful display of the circulation, both in its 

 external gills and in its delicate feet. It may be inclosed in a large 

 aquatic box or in a shallow cell, gentle pressure being made upon 

 its body, so as to confine its movements without stopping the heart's 

 action. The circulation may also be seen in the tails of small fish, 

 sucl i as the minnoiv or the stickleback, by confining these animals in 

 tubes, or in shallow cells, or in a large aquatic box ; but although 

 the extreme transparence of these parts adapts them well for this 

 pin-pose in one respect, yet the comparative scantiness of their 

 blood-vessels prevents them from being as suitable as the frog's web 

 in another not less important particular. One of the most beautiful 

 of all displays of the circulation, however, is that which may be seen 

 upon the yolk-bag of young fish (such as the salmon or trout) soon 

 after they have been hatched ; and as it is their habit to remain 

 almost entirely motionless at this stage of their existence, the obser- 

 vation can be made with the greatest facility by means of the 

 zoophyte-trough. The store of yolk which the yolk-bag supplies 

 for the nutrition of the embryo not being exhausted in the fish (as 

 it is in the bird) previously to the hatching of the egg, this bag 

 hangs down from the belly of the little creature on its emersion, 

 and continues to do so until its contents have been absorbed into 

 the body, which does not take place for some little time after- 

 wards. And the blood is distributed over it in copious streams, 

 partly that it may draw into itself fresh nutritive material, and 

 partly that it may be subjected to the aerating influence of the 

 surrounding water. 



The tadpole serves, moreover, for the display, under proper 

 management, not only of the capillary, but of the general circulation ; 

 and if this be studied under the binocular microscope, the observer 

 not only enjoys the gratification of witnessing a most wonderful 

 spectacle, but may also obtain a more accurate notion of the rela- 

 tions of the different parts of the circulating system than is other- 

 wise possible. The tadpole, as every naturalist is aware, is 

 essentially a fish in the early period of its existence, breathing by 

 gills alone, and having its circulating apparatus arranged accord- 

 ingly ; but as its limbs are developed, and its tail becomes relatively 

 shortened, its lungs are gradually evolved in preparation for its 

 terrestrial life, and the course of the blood is considerably changed. 

 In the tadpole as it comes forth from the egg the gills are external, 

 forming a pair of fringes hanging at the sides of the head (fig. 792, l) 

 and at the bases of these, concealed by opercula or gill-flaps 



1 A special form of live-box for the observation of living tadpoles &c., contrived 

 by Prof. F. E. Schulze, is described and figured iu the Quart. Journ. Microsc. Sci. 

 nls. vol. vii. 1867, p. 261. 



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