736 CIRCULATION OF BLOOD IN FISH. 



less; this usually happens when it has been raised to a temperature 

 between 100° and 110°; 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.* — 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 

 enclosed in a large Aquatic-box or in a shallow Cell, gentle pressure 

 being made upon its body so as to impede its movements without 

 stopping the heart's action. — The Circulation may also be seen in 

 the tails of small Fish, such as the Minnow or the Stickleback, by 

 confining these animals in tubes, or in shallow cells, or in a large 

 Aquatic-box ;f but although the extreme transparence of these 

 parts adapts them well for this purpose 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 Cir- 

 culation, 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 motion- 

 less at this stage of their existence, the observation can be made 

 with the greatest facility by means of the Zoophyte-trough, provided 

 that the subject of it can be obtained. Now that the artificial 

 breeding of these Fish is largely practised for the sake of stocking 

 rivers and fish-ponds, there can seldom be much difficulty in pro- 

 curing specimens at the proper period. 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 happen 

 for some little time afterwards. 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. 



576. The Tadpole serves, moreover, for the display, under proper 

 management, not only of the Capillary but of the general Circula- 

 tion ; 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 relations of the different parts of the Circulating System 

 than was previously possible. + The Tadpole, as every Naturalist is 



* A special form of Live-box for the observation of living Tadpoles, &c, 

 contrived by F. E. Schultze, of Rostock, is described and figured in the 

 "Quart. Journ. of Microsc. Science," N.S., Vol. vii. (1867), p. 261. 



t A convenient Trough for this purpose is described in the "Quart. 

 Journ. of Microsc. Science," Vol. vii. (1859), p. 113. 



X See Mr. Whitney's account of "The Circulation in the Tadpole,' in 



