LARV^ AND METAMORPHOSES 19 



morphosis and a direct development. In most of them the eggs are 

 laid in water and true tadpoles hatch out. In some the eggs hatch 

 on land, having been laid in holes, on grass or leaves, and when 

 the tadpoles are hatched, they wriggle into water or are washed 

 into pools by the rain. In others, again, the eggs are laid on land, 

 and the tadpoles have lost their giUs before they are hatched, but 

 the metamorphosis is completed later on. In a few the complete 

 change occurs inside the egg, and when hatching takes place 

 little frogs appear, sometimes, however, with a stump of the tail 

 still left. In others the eggs are carried by the parent, and here, too, 

 they may be hatched as tadpoles or as perfect frogs. It would be 

 difficult to find a better example of the gradual change from a 

 type of development which is a repetition of the ancestral history, 

 to the higher type in which the young, as soon as they assume active 

 life on their own account, resemble their parents more or less closely. 

 The metamorphosis of the tadpole into the frog is a change from 

 a lower to a higher type of life. The larvae of ascidians or sea -squirts 

 change by metamorphosis into an adult which must certainly be 

 regarded as a lower form of life. The eggs hatch into small tadpoles 

 which swim actively through the sea by vibrating the webbed 

 tail, the latter being stiffened by a simple kind of backbone in the 

 form of a rod of tough jeUy. There is a hoUow spinal cord, rather 

 like that in the very young tadpole of a frog, and in the front of this, 

 in the region where the brain of the frog's tadpole is developed, 

 there is a simple kind of eye and ear. Near the mouth there 

 'are adhesive organs by which the creature can anchor itself tem- 

 porarily. The mouth leads into a wide guUet pierced by giU-shts, 

 some of which at least correspond with the giU-slits of the frog's 

 tadpole. At the metamorphosis, the larva fixes itself permanently, 

 at first by the adhesive organs, and afterwards by an outer jacket 

 or test which covers the whole animal with a protecting coat. The 

 tail with its representative of the backbone, the greater part of the 

 nervous system, and the sense-organs, disappear. The guUet and 

 'the part of the body surrounding it increase in size, until they make 

 up the greater part of the bulk of the animal. The wall of the 

 gullet becomes transformed into a sieve, pierced by innumerable 

 holes through which the sea-water is filtered, leaving behind the 

 small particles which are used as food. The active, swimming 

 larva (Fig. 9), with a structure extremely hke that of the lower 

 vertebrates, changes in this way into a hoUow bag which sucks in 

 water by one hole and pours it out by another, and which, if we 



