THE BREEDING BEHAVIOR OF ANIMALS 



l91 



The common song birds are all altricial, while domestic fowls, partridges, 

 most wading birds, and the various ducks are precocial. 



There are also animals which escape from the egg so early that they 

 lack important organs and must undergo extensive changes to attain 

 the adult form. Or they may possess 

 organs which they must lose before *they 

 become adults. Young animals, leading a 

 separate existence but lacking certain organs 

 of the adult or possessing organs not found 

 in the adult, are known as larvae. The 

 offspring of jellyfishes emerge from the ovary 

 of the mother, where in some kinds as stated 

 earlier the eggs are fertilized, as a simple 

 ball of cells, almost at the beginning of 

 development. They receive no care what- 

 ever thereafter. The embryos of sponges 

 escape at a stage almost as early, as the 

 jellyfishes. The developing embryos of 

 starfishes, sea urchins and their allies (Fig. 

 162), and marine worms are also capable of 

 free-swimming existence at a very early 

 stage. In the frogs and toads the tadpole is a larval form (Fig. 163), but 

 it hatches at a much later developmental stage than do the larvae of the 

 several preceding examples. 



Early development, may be direct or indirect. In direct develop- 

 ment the embryo develops directly toward the sexually mature condition, 

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



Fig. 162. — Free-swimming 

 larva of the holothurian Syn- 

 apta, leading an active inde- 

 pendent existence at a very 

 early stage of embryonic de- 

 velopment. 



Fig. 163. — Tadpole of frog, illustrating a larval form. Organs are present that are lacking 

 in the adult, and some organs are missing which the adult possesses. 



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

 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. 



