DEVELOPMENT 189 



which are small and are not provided with a large store of nutritive 

 materials. The egg very often develops when floating freely in 

 the sea, or other aquatic medium, or when lying on the sea bottom, 

 etc. Development proceeds more rapidly than in the cases we 

 have already mentioned, for the presence of food-yolk and other 

 nutritive materials in the large-yolked egg impedes the process 

 of segmentation of the ovum. But the development generally is 

 arrested, or pauses, in the small-yolked eggs and what hatches 

 from the egg-envelope, cocoon, etc., is not a creature with the 

 morphological characters of the adult parent but a larva, which 

 is usually very different in structure and habit from the parent. 

 Very often the larva is an organism that is mobile, that can find 

 its own food and is autonomous and freely living. It does not 

 reproduce. For a time it lives independently of the parent. After 

 this first larval phase, and when the organism has grown to some 

 extent, development begins again in an accelerated manner. A 

 '' metamorphosis " is said to occur and the creature may rapidly 

 take on the structure and habits of the parent so that it recogniz- 

 ably belongs to the same species as the latter. Or the meta- 

 morphosis may result in the appearance of another larval phase 

 when the organism is still different in structure and habit from 

 the parent and there may be three or four such larval phases in 

 an indirect development. A good example is that of the common 

 Barnacle which has the following life-history : 



First phase. A creature called the '' Nauplius " larva hatches out 

 from the egg ; 



Second phase. After living in the sea and growing the nauplius 

 undergoes metamorphosis into the " Cypris " larva ; 



Third phase. After a free life in the sea the Cypris metamorphoses 

 into the adult form which then attaches itself to the sea 

 bottom and rapidly changes into the Barnacle. 



This is an example of a very great number of life-histories. 

 Often, and particularly in the cases of animals which live parasit- 

 ically in, or on the bodies of others the indirect, or discontinuous 

 development may be a very complicated process. We need not 

 go into details of such life-histories. From the point of view of 

 the natural selection hypothesis their significance inheres in the 

 opportunities for obtaining nutriment apart from the parent, or 

 from the egg emitted by the latter ; for being widely distributed 



