150 GENERAL PRINCIPLES OF ZOOLOGY 



germ-layers) are going on the young animals are usually enclosed within a 

 firm protective covering, or even in the maternal sexual apparatus (uterus), 

 and are hence called embryos. Later stages, even the formation of the most 

 important organs, may occur during embryonic life, as we see in many ver- 

 tebrates, worms and crabs, which, at the end of their embryonic existence, 

 are complete in all their parts, and need only the maturity of the sexual 

 organs, and growth of the body as a whole, in order to reach the climax of 

 their development. On the other hand, there are animals, chiefly aquatic, 

 which, after leaving the egg, undergo important changes, like the ccelen- 

 terates, echinoderms, insects, amphibians, etc. The ccelenterates, echino- 

 derms, and many worms usually escape from the egg even before the forma- 

 tion of the germ-layers, and, as free-swimming ciliated planula, form the 

 germ-layers and organs. Since there is here a more or less extensive post- 

 embryonic development, it is a misnomer to apply the term 'embryology ' to 

 both stages; it is" necessary, rather, to limit the name to the developmental 

 processes inside the egg, and, on the other hand, to speak generally of the 

 history of the development of the individual, or ontogeny. As the unde- 

 veloped animal within its membrane is called an embryo, so the name larva 

 is applicable to the free-living but not mature animal. 



Direct and Indirect Development Metamorphosis. Larval de- 

 velopment may be either direct or indirect. In direct development, as 

 the term implies, the larva pursues a straight course towards the sexually 

 mature animal, the lacking organs being outlined one after another; 

 hence it is continually becoming more like the sexually mature animal. 

 In indirect development, on the contrary, organs belonging only to the 

 larval life, larval organs, are formed and later are destroyed. Therefore 

 in the definition of indirect development, or metamorphosis, emphasis is 

 laid upon the presence of larval organs. Thus caterpillars are distin- 

 guished from butterflies not only by the absence of compound eyes and 

 wings, but by the presence of anal feet and spinning-glands, and further 

 by the different shape of the jaws, antennae, and legs, the different arrange- 

 ment of the tracheae and nervous system, etc. Tadpoles are distinguished 

 from frogs not only by the absence of lungs and extremities, but also by 

 the presence of gills and tail. The more numerous the larval organs, the 

 more pronounced will be the metamorphosis. 



Neoteny. Occasionally the gonads of an indirectly developing animal 

 become mature before the metamorphosis is complete, and as a result develop- 

 ment is brought to a standstill (sexually mature, gill- breathing Triton larvae, 

 larvae of Miastor, etc.). This peculiarity in which the gonads restrict the devel- 

 opment, is called 'neoteny'; and the attempt is made to regard a number of 

 species as neotenic, that is sexually ripe in the larval stages. 



