THE BREEDING HABITS OF ANIMALS 



203 



also capable of free-swimming existence at a very early stage. In the 

 frogs and toads, the tadpole is a larval form (Fig. 159), but it hatches at a 

 much later developmental stage than do the larvae of the several pre- 

 ceding examples. 



Larval development may be direct or indirect. In direct development 

 the larva develops directly toward the sexually mature condition, the 

 organs being outlined and developed one after the other. In indirect 

 development, on the contrary, organs belonging only to the immature 



Fig. 157. — Recently hatched young of the chimney swift, Chcetura pelagica (Linnaeus) 

 (left), and spotted sandpiper, Actitns macularia (Linnaeus) (right). These are examples, 

 respectively, of altricial and precocial birds. The chimney swift is blind and naked when 

 hatched (altricial) ; the sandpiper is covered with down and the eyes are functional (pre- 

 cocial). 



stages and for that reason called larval organs are first formed and later 

 destroyed. Thus the caterpillars (larval stage) of butterflies are dis- 

 tinguished from the adult not only by the absence of wings and compound 

 eyes but also by the presence of anal feet and spinning glands which are 

 absent in the adult butterfly; and tadpoles of toads and frogs (Fig. 

 159) are distinguished from the adult frog not only by the absence 

 of kmgs and legs but also by the presence of gills and tail. The trans- 

 formation by which the larval organs disappear and the missing organs 

 are constructed is known as metamorphosis. The more numerous the 



