ANIMAL BIOLOGY. [Part I. 



The embryo is now getting ready to leave the egg within 

 which it lies, with its tail somewhat curved to one side (ii.). At 

 last it becomes more active, breaks through the jelly-like mass 

 and is hatched. It soon attaches itself to plants, or to the out- 

 side of the remaining gelatinous material, by means of the pair 

 of suckers (5.) near the mouth, and somewhat later enters on a 

 free-swimming existence. There are now (iii. ex. 5r.) three pairs 

 of external branchiae. In front of the first, and behind the first 

 and second, there are slits or gill-clefts passing inwards to the 

 throat. Water is taken in at the mouth and passes out through 

 the clefts just as in a fish. Through the branched filaments of 

 the gills the circulation of the blood may be watched under the 

 microscope. 



As the fish-like tadpole enlarges, a membranous fold (oper- 

 culum, iv., op.) is developed in front of the gills, and gradually 

 extends backwards over them. Before long it has completely 

 covered the external gills on the right side, and closely adheres 

 to and unites with the body behind the clefts. Thus the exit of 

 water on the right side is stopped. On the left side, however, 

 the fold does not altogether unite with the body, but leaves a 

 branchial aperture (v., br. a.) which remains until near the end of 

 tadpole life. Beneath the opercular membrane on each side 

 there is a branchial chamber, and the two chambers communicate 

 below. But the water which passes through both chambers, 

 makes its exit through the single branchial aperture. During 

 the formation of the branchial chambers the external gills 

 atrophy, that is, disappear by absorption ; but to take their place 

 internal gills are developed on the inner sides of the branchial 

 clefts. 



Meanwhile the tadpole has become an active and vigorous 

 feeder, mostly vegetarian, but by no means despising animal 

 food. The mouth (v., m.) lies at the bottom of a somewhat pro- 

 trusible cup, with a circular lip covered over with horny 

 asperities, while within the cup the mouth is armed with two 

 great horny crescentic jaws, with which it crops the water weeds, 

 or browses on the flesh of some dead comrade. 



The first sign of limbs is the appearance of two little rudi- 



