134 THE BOOK OF NATURE STUDY 



testine becomes much shorter ; the gills and gill-arches disappear 

 (though parts of the gill-arches are utilised) ; the heart is now 

 three-chambered ; the lungs become the sole breathing organs 



(apart from the skin, which is 

 always respiratory in Amphib- 

 ians), and the young frogs, with 

 a minute stump of a tail, leap 

 ashore. They will drown in the 

 aquarium if they are not allowed 

 to leave the water. 



FIG. 47. A stage between tadpole and 



small frog, about three months after Retrospect ES to Food. (l) At 



hatching, The metamorphosis is nearly first the niOUtllleSS embryo SUb- 



complete but part of the tail remains sists Qn ^ store Qf ^ ^ ^ 



unabsorbed. J x ' 



begins to eat minute organisms 



and particles in the muddy ooze. (3) It begins to attack 

 larger animals and pieces of larger plants. (4) It has a fast- 

 ing period about the time of metamorphosis, during which 

 the supplies afforded by the tail are utilised in construction. 

 (5) It is, as an adult, in the main insectivorous. If food 

 is not abundant tadpoles devour other tadpoles, and the 

 consequent reduction in the numbers kept in the aquarium is 

 very familiar. In their later stages tadpoles may be utilised to 

 clean up delicate skeletons, e.g. of a shrew or of a bat. Frank 

 Buckland notes that in their most intense carnivorous period 

 tadpoles will nibble at the toes of little boys who wade in the 

 pools. 



Retrospect as to Breathing. (i) At first the gill-less tadpoles 

 breathe through their skin, which is the most primitive mode of 

 respiration, seen in animals like earthworm and leech, but en- 

 tirely lost in animals higher than Amphibians. (2) Then there 

 are three external gills. (3) These are replaced by a second 

 set " internal " gills, which are really continuous with the first 

 set. (4) Gills and lungs function together when the tadpole 

 is about two months old, a state of affairs which may be com- 

 pared with that in the double-breathing mud-fishes or Dipnoi, 

 which have " lungs " as well as gills. (5) The adults breathe 

 by lungs. (6) In the winter-rest the respiration is again wholly 

 cutaneous. Cautiously and critically the teacher should try to 



