THE COMMON FROG. 51 



The water is sometimes squirted with considerable force five or 

 six inches. Hence their name " Sea-squirts." They are not 

 infrequent along our coasts. In these Ascidians we have a re- 

 markable exception to the rule that larva develop into higher 

 organisms. They compel biologists to seek elsewhere for the 

 origin and subsequent decadence of the median eye. 



The frog, on the contrary, continues to improve in organisation 

 until it reaches maturity in its third summer. Its respiratory and 

 other organs are all perfected, also its bony skeleton, at first 

 mere cartilage. Parts of the skull become hardened, or are re- 

 placed by bony pieces. In the illustration of Rana escuknta 

 (p. 59) this may be seen ; it differs in no respect from R. 

 temporaria. 



The chief characteristics of the genus Rana are teeth in the 

 upper jaw and on the palate (the latter are called vomerine teeth 

 from the supposed resemblance of the bone in which they are 

 fixed to a plough, vomer\ pupil of the eye horizontal, fingers free, 

 toes webbed, hind legs very long and formed for leaping, tongue 

 notched and free behind, skin smooth, moist, extremely sensitive 

 and an organ of respiration. Frogs, therefore, have a double 

 respiration, swallowing air by means of what Mivart calls "an 

 effective throat air-pump," and breathing through their skin as 

 well. Experiments have proved that the skin answers all the 

 purposes of lungs. A bladder tied so tightly over the head of a 

 frog as to entirely exclude air does not incommode it. Placed in 

 water in this condition, and kept there for a time, the water is 

 found to contain carbonic acid given off through the skin, just as 

 we give off carbonic acid from our lungs in each expiration. On 

 account of this sensitive skin and the active perspiration by which 

 it is kept moist, frogs in the vivarium must not be exposed to the 

 sun glare in hot weather. Unmindful of the importance of shade, 

 I once left the glass bowl in which my very young frogs resided 

 close to a window exposed to the full glare of a July sun. All 

 had seemed apparently well. that morning, but on looking at them 

 in the afternoon one was dead. It could, like the rest, have 

 sought shelter under or behind the minerals or green sprays with 

 which their home was furnished, but this little frog was much 

 given to climbing up the side of the bowl and clinging there, and 

 I have no doubt it had been struck by the intensely hot sun ot 

 that day. 



The bony structure and muscles of a frog are extremely like 

 our own, only modified to suit its requirements. In the skeleton, 

 (fig. 22) you see how admifably the powerful hind limbs are adapted 

 for leaping. It is common to say of a frog, as of a grasshopper, 



