192 PRINCIPLES OF ANIMAL BIOLOGY 



163) are distinguished from the adult frog not only by the absence of lungs 

 and legs but also by the presence of gills and tail. The transformation 

 by which the larval organs disappear and the missing organs are con- 

 structed is kno^vn as metamorphosis. The more numerous the larval 

 organs the more pronounced the metamorphosis becomes. This phe- 

 nomenon is further described in Chap. 16. 



Relation of Birth Stages to Parental Care. — That birth at an early 

 stage of development necessitates parental care would seem at first con- 

 templation to be obvious. That is not usually true, however, except 

 for the animals of common daily observation. It cannot be said for 

 animals in general that the stage of development at birth determines 

 the amount of parental care necessary, for many of the lower invertebrates 

 with incomplete larvae and many fishes which have very immature young 

 give no care to the offspring, while other invertebrates with feeble young 

 (for example, the ants) carefully guard and feed them. But it is note- 

 worthy that, where no care is exercised, the young born in early stages are 

 usually those of aquatic or amphibious forms, while the young of ter- 

 restrial forms are mostly born in relatively advanced stages or receive 

 parental care. Furthermore, while many aquatic forms give some atten- 

 tion to the young, it is among the terrestrial forms that the greatest 

 development in the habit of caring for the offspring is found. It may 

 thus be concluded that, when aquatic animals, or amphibious forms 

 with aquatic young, deposit the eggs or young in suitable habitats, they 

 have done much to facilitate postembryonic development, but that 

 land forms must usually give birth to young in an advanced stage of 

 development or exercise parental care in proportion to the helplessness 

 of the offspring. 



