2QO 



TADPOLES OF THE SEA 



and disappears, leaving no trace behind it, and with 

 it disappear the elastic backbone and the spinal nerve- 

 cord. The brain remains, but loses its eye, and does 

 not grow so much in proportion as do other parts 

 (Fig. 38). It is the gullet which increases most ; it 

 becomes relatively enormous, and the few perforations 

 or gill-slits in its wall become multiplied so as to form 

 a regular net-like trellis-work. The skin deposits a 

 thick and translucent coat or " tunic " all over the surface 

 of the body, leaving a hole for the mouth and another for 

 the pore or opening of the peri-branchial chamber. Thus 

 protected and fixed in a safe place the tadpole trans- 

 formed into a sea-squirt grows as a featureless oval sac to 

 a length of two or three inches. It eventually produces 

 eggs, which grow into little tadpoles and swim away 

 into the sea, and so the round of its life is completed. 



The Ascidian or sea-squirt is specialised for feeding 

 and breathing by means of the current of water passing 

 into its mouth and out by its gill-slits. It has no need 

 of movement nor of eyes nor ears ; it leads a passive, 

 unaggressive life, and is distinctly inferior, in its general 

 capability for dealing with the surrounding world, to the 

 little tadpole by the change of which it is formed. 



Now let us compare with this the history of the other 

 tadpole that of the frog (Fig. 39). It takes in food by its 

 mouth, and soon develops movable horny lips and a jaw, 

 which enable it to nibble and feed on soft solid food. It 

 grows to an inch or more in length, keeping its large, mus- 

 cular tail and fish-like appearance. Then four legs appear, 

 first the hind pair and then a front pair. The tail ceases 

 to grow, and withers, whilst the legs and the rest of the 

 body increase. At last the tail is a mere projecting stump, 

 and the tadpole crawls out of the water and becomes a 

 little frog. The legs are strong, muscular limbs used for 

 crawling and jumping, and for swimming when the young 



