204 



PRINCIPLES OF ANIMAL BIOLOGY 



larval organs the more pronounced the metamorphosis becomes. This 

 phenomenon is further described in Chapter X. 



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

 stage of development necessitates parental care would seem at first 

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

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



animals as a group that the stage of develop- 

 ment at birth determines the amount of 

 parental care necessary, for many of the 

 lower invertebrates with incomplete larvse and 

 many fishes which have very immature young 

 give no care to the offspring, while other in- 

 vertebrates with feeble young (for example the 

 ants) carefully guard and feed them. But it 

 is noteworthy that where no care is exercised 

 the young born in early stages are usually 

 those of aquatic or amphibious forms, while 

 the young of terrestrial forms are mostly born 

 in relatively advanced stages or receive 

 parental care. Furthermore, while many 

 aquatic forms give some attention 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 post-embryonic development; but that 

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



Fig. 158. — Free-swimming 

 larva of the holothurian 

 Synapta, leading an active in- 

 dependent existence at a very 

 early stage of embryonic de- 

 velopment. (Fro7n drawing by 

 P. O. Okkelberg.) 



Fig. 159. — Tadpole of frog, illustrating a larval form. Organs are present which are 

 lacking in the adult, aui sonic organs arc missing which the adult possesses. 



development, or exercise parental care in proportion to the helplessness 

 of the offspring. 



Conclusion. — The brief analysis just given shows that the breeding 

 liabits of animals are very varied, that th(^y bear some relation to the 

 mode of life, and that there is no general evolutionary sequence in the 



