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NATURAL HISTORY. 



in the method and structure of respiration and circulation, and also in the organs of motion, special 

 sense, and digestion. So that during the earlier lifetime, an incomplete and altogether differently- 

 shaped creature is being perfected into a permanent adult form. This process is called metamorphosis. 

 In the early days of Tadpole life much of the organisation (circulation and respiration) is very like that 

 of the fish, for the simple heart of these last is furnished to the little things. '1 here are minute 

 thread-like gill-fringes, or branchiae, just behind the head, and they only require a heart which will 

 supply them a single or branchial heart. But as growth proceeds, the branchiae become 

 hidden in a cavity, and then are absorbed, the lungs growing within, and a heart which has a 

 double nature, and which relates to the body and also to the lungs, being systemic and pulmonic, 



is gradually developed. The limbs are 

 at first scarcely perceptible, and become 

 gradually developed, passing through a 

 rudimentary stage beneath the skin, from 

 which they do not emerge until they 

 have attained considerable size and a 

 definite shape. The hind legs appear first, 

 and they are soon employed to assist in 

 a feeble manner the strong and active 

 tail in moving about. The tail is 

 developed to a great degree, and it is 

 made up of muscles surrounding vertebrae, 

 which form a long column, but they are 

 not ossified (in those Amphibia in which 

 the tail persists the vertebrae are ossified 

 STRUCTURE OF THE TADPOLE. early). As weeks pass on the limbs 



(A) Side v-iew.showingGHls, g; Mouth, m; Nasal Sae.n; Eye, e; Ear, a; ;B> from below, i ,-> , , i i > 



ss, Suckers ; o, Operculum ; /, Horny Jaws ; (c) More advanced Tadpole, showing gl'OW, and tile tail aimilllSlieS by abSOrp- 



growth of Ojierculum, so as to enclose (Jills, save at opening 6 on left side: hi, rudi- . 



mentof hind limbs; , Sucker; ID) Head of young Tadpole (magnified); gi. j,2, tlOll, and gets Smaller and Smaller, Until 

 external bills ; Ip, Upper Lip ; cl l to cte, Clefts. 



it disappears. It does not drop off, but 



its substance is received grain by grain into the adult animal. The skull, very cartilaginous at 

 first, becomes consolidated and bony to a considerable extent, and thus is more reptilian than 

 fish-like. 



The branchiae at first appear in about fifty hours, when the temperature is warm, as small 

 projections, and shortly afterwards a " holder " appears on either side of the future mouth. Before 

 the fourth day in hot places, and in England not for a month, it emerges, jerks itself about, and 

 breaks out of the egg a free swimmer. 



The branchiae consist of two principal divisions, or branches, from each of which proceed four 

 or five leaf-like processes subdivided into numerous little filiform leaflets regular in shape, and forming 

 the ultimate divisions of the structure. On these ramify the minute capillary blood-vessels, and the 

 blood undergoes its change there, being oxygenated, and evolves carbonic acid gas. A minute branch 

 of an artery conveys the impure blood from the heart and enters each leaflet at its base, and passes 

 along its shorter or inner margin, giving off capillary branches in its course, which, after meandering 

 over the surface of the leaflets, and communicating with each other in various directions, pass over to 

 the opposite side, and join and form a branchial vein. This unites with others at the base of the 

 leaflets, and thus a vessel is formed which takes the purified blood to the heart. 



In this stage the circulation of the blood may be seen, under the microscope, to perfection in the 

 gills. The current of the blood passes up each stem of the branch, and a distinct stream is given off to 

 each leaf. It is propelled to the end, and returns down the opposite sides by the veins. Every 

 blood corpuscle red and white is visible, and they move in the transparent vessels with singular 

 regularity. As growth proceeds the branchiae become fully developed, and then they begin to diminish 

 in size, become obtuse, and are gradually so reduced as to be withdrawn within the branchial cavity, 

 and concealed by a little cover, or operculum, of the integument. It must be noticed that as soon as 

 the true Tadpole shape is assumed, and the branchiae are within their cavity, and in communication 

 with the outside through certain slits in the neck, they are supplied with water, which enters the 



