150 GENERAL PRINCIPLES OE ZOOLOGY 



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

 I'lrm protective co\ering, or cveir in the maternal sexual apparatus (uterus), 

 and are hence called cvihryos. Later stages, e\cn the formation of the most 

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

 tebrates, worms and cralis, A^hich, 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 bodv as a whole, in oriler to reach the climax of 

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

 which, after leaving the egg, undergo important changes, like tlie caden- 

 terates, eclrinoderms, insects, amphibians, etc. The ccelenteratcs, cchino- 

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

 tioii of the germ-layers, and, as free-swimming ciliated planukr, 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 'eml )ryology ' to 

 both stages; it is necessary, rather, to hmit 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 williin 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. 1\\ 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 tire sexually mature animal. 

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

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

 in the del'mition of indirect development, or inelaniorphosis, emphasis is 

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

 guished from butterllies not only by the absence of compound eves and 

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

 by the different shape of the jaws, antenna^ and legs, the different arrange- 

 ment of the trachea- 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 pronounceil will be the metamorphosis. 



Neoteny. — Occasionally the gonads of an indirectly develo]iing animal 

 become mature before the metamorphosis is complete, and as a result de\elop- 

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

 larva; of iliaslor, etc.). This peculiarity in which the gonads restrict the devel- 

 opment, is called 'neoteny'; and the attem]it is made to regard a number of 

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



