200 



THE POPULAR EDUCATOR. 



afterwards fecundated. The resulting young are called tad- 

 poles. In their early stage they resemble fishes. They breathe 

 by means of gills, which project from each side of the body 

 behind the head. (Fig. II.) They have no fins, and in their 

 early stage they are destitute of legs. (Fig. I., a.) As life ad- 

 vances these external gills disappear, the animal breathing by 

 means of internal gills, which are suspended from arches, and 

 bathed by the water in a similar manner as that arrangement 

 described in fishes. Presently a pair of legs (Fig. I., b) may be 

 seen to grow from the sides of the body. The hind legs make 

 their appearance first, and the fore legs subsequently, in the 

 frog. (Fig. I., c.) This is not always the case with the other 

 Amphibia ; for example, in the salamander the order of leg- 

 appearance is reversed. In the siren the hind legs are wanting. 

 As the legs approach to- 

 wards a state of perfect de- 

 velopment, the tail gradually 

 contracts and wastes (Fig. 

 I., d), until it has completely 

 disappeared. During this 

 period changes are taking 

 place in the internal as well 

 as external economy of the 

 body. Nature now prepares 

 it for an extended sphere of 

 action by endowing it with 

 a pair of lungs, by which it 

 is enabled to live either in 

 its native element, or to ex- 

 tend its peregrinations to 

 terrestrial soil, and live there 

 also. This transition from 

 the larval to the frog con- 

 dition cannot fail to remind 

 the student of another me- 

 tamorphosis namely, that 

 which the caterpillar un- 

 dergoes to become butterfly 

 or moth. In the former 

 the transit is from a strictly 

 aquatic to a double form of 

 life ; in the latter from an 

 earthy to an aerial state of 

 existence. It is by such 

 metamorphoses as these 

 that Nature teaches man to 

 aspire to a higher degree of 

 intelligence and usefulness. 

 The lesson comes with an 

 equal force from the much- 

 despised toad whose hoarse 

 croakings break the still- 

 ness of the night in its quiet 

 reign of darkness over their 

 marshy habitations as it 

 does from the pretty but 

 irresolute butterfly, bask- 

 ing to and fro in the sun- 

 shine of day. In the frogs, 

 toads, and newts the gills 



entirely disappear, and for this reason they have been named 

 Caducibranchiate Amphibia.* Others are called Perenni- 

 branchiate Amphibia, from the fact that their gills remain 

 permanently, even after the formation of complete lungs. 

 Such are the proteus and eiren ; also the axolotl, to which 

 the Mexicans are partial as an article of diet, especially 

 when, as Dr. Baird remarks, dressed after the manner of stewed 

 eels, and served up with rich and stimulating sauces. 



The Circulatory Apparatus. The heart of the Amphibia is 

 indicative of progressive development. It consists of three 

 chambers or cavities. Two of these are reception cavities, and 

 named the systemic and pulmonic auricles ; the third is a pro- 

 pelling one, and called the ventricle. The object of the ventricle 

 is to propel the blood to the system and lungs to the system 

 for the purpose of carrying oxygen for the nutrition of the 

 tissues, and to the lungs so that the oxygen element may be 



* From cutZu-cus, easily falling ; branchuE, gills. 



AMPHIBIA. I. (a, b, c, d) SUCCESSIVE METAMORPHOSES or THE FROG. II. 

 TADPOLE OF FROG, SHOWING EXTERNAL GILLS. III. SKELETON OF THE 

 FROG. IV. BLOOD-VESSELS OF TADPOLE OF FROG, AND THEIR MODE OF 

 DISTP.IBtmON TO THE GILLS. V. BLOOD CORPUSCLES OP THE FROO 

 (HIGHLY MAGNIFIED). 



Eefs. to Nos. in Figs. IV. 1, artery arising from a single ventricle, and di- 

 viding into six branches, which go to the three pairs of gills, 2 Z, 3 3', 4 4'. 



again restored to it from the atmosphere, and to expel from the 

 blood the carbonic acid which results from the waste products. 



It will be surmised that in those animals (for example, the 

 frog, etc.) possessing only temporary gills, that, as the lungs 

 usurp their place, a change must of necessity arise in the 

 arrangement of the blood-vessels. This is the case. When the 

 lungs come into play, the blood is diverted to them and away 

 from the gills. (Fig. IV.) In those Amphibia with persistent 

 gills this change is only partial. In the frog tribe the skin also 

 acts as an organ of respiration by absorbing moisture. By 

 reason of this it is enabled to live for a long time deprived of 

 food and air. This fact has given origin to many amusing tales 

 of toads being found alive entombed in coal-beds and blocks of 

 stone, where they had evidently existed (believe it who chooses ! ) 



for hundreds of years. 



The digestive and nervous 

 apparatus undergo a slight 

 increase in complexity from 

 that described in the last 

 lesson. 



Frogs are destitute of 

 ribs, and consequently have 

 not an expansile chest. This 

 necessitates them to breathe 

 by swallowing the air. The 

 skeleton of the Amphibia, 

 evinces decided advances 

 towards that of the higher 

 Vertebrata. This is very evi- 

 dent in the disposition and 

 conformation of the bones 

 of the limbs i.e., in those 

 which possess the latter. 

 The skull joins with the ver- 

 tebral column by means of 

 two condyles, which, Profes- 

 sor Huxleyremarks, sharply 

 distinguishes the Amphibia 

 from the higher Vertebrata. 



Far away beyond the con- 

 fines of history probably 

 ages before the secondary 

 organisation the earth was 

 tenanted by gigantic spe- 

 cies of the class Reptilia. 

 The reorganisation of the 

 earth completed, and man 

 placed upon its surface, we 

 find the reptile again play- 

 ing a prominent part. Scrip- 

 ture debits the snake with 

 the credit of inducing our 

 first parents to commit the 

 sin of disobedience. Known 

 from the earliest times, 

 their ungainly appearance, 

 their malignity of disposi- 

 tion, and the formidable 



attribute (poison-fangs) peculiar to an order of this class, have 

 rendered them objects of hatred and fear. They are regarded 

 by every one except the enthusiastic naturalist as the most 

 despicable part of the whole of Nature's handiwork. Shakespeare 

 drew from them an expressive illustration of dissimulation : 



" And Gloster's show 

 Beguiles him, as the mournful crocodile 

 With sorrow snares relenting passengers ; 

 Or, as the snake, rolled in a flowering bank, 

 "With shining checkered slough, doth sting a child, 

 That, for the beauty, thinks it excellent. 



The Eeptilia are now classified with birds as a sub-group of 

 Vertebrata, which Professor Huxley calls the Sauropsida.* 



In external appearances and configuration the orders of this 

 class differ materially from each other. The Crocodilia have 

 their bodies covered with horny plates embedded in the skin. 



Zavpo, a lizard ; o\fr<t, appearance. 



