DEVELOPMENT. 67 



animal which is fixed in the adult state may be provided with 

 largely-developed locomotive organs; while that of an adult 

 which feeds by suction may be provided with powerful appa- 

 ratus for the seizure and manducation of vegetable and ani- 

 mal prey. 



The larva of a free adult may be parasitic, or that of a 

 parasitic adult free and actively locomotive. Moreover, the 

 whole course of development maj^ take place outside the body 

 of the parent, or more or less extensively within it ; whence 

 the distinction of oviparous, ovoviviparous, and viviparous 1 

 animals. 



Finally, when development takes place within the body of 

 the parent, the foetus may receive nourishment from the latter 

 by means of an apparatus termed a placenta, by which an 

 exchange between the parental and foetal blood is readily 

 effected. Examples of placentae are found not only in the 

 higher mammals, but in some Plagiostome fishes and among 

 the Tunicata. 



In many insects and in the higher Vertebrates, the em- 

 bryo acquires a special protective envelope, the amnion, 

 which is thrown off at birth ; while, in many Vertebrates, 

 another foetal appendage, the allantois, subserves the respi- 

 ration and nutrition of the foetus. 



The strange phenomena included under the head of the 

 "Alternation of Generations," and which result from the di- 

 vision, by budding or otherwise, of the embryo which leaves 

 the egg, into a succession of independent zooids, only the last 

 of which acquires sexual organs, have already been gener- 

 ally discussed. 



IV. — THE DISTRIBUTION OF ANIMALS. 



The distribution of animals has to be considered under 

 two points of view : first, in respect of the present condi- 

 tion of Nature ; and secondly, in respect of past conditions. 

 The first is commonly termed Geographical, the second 

 Geological, or Paleontological, Distribution. A little con- 



1 As eggs capable of development are alive, this terminology is etymologi- 

 cally bad ; and ovoviviparous is particularly objectionable, as all animals bring 

 forth live eggs, or that which proceeds from them. But, as understood to ap- 

 ply to animals which lay eggs, to those in which the eggs are hatched within 

 the interior of the body without any special foetal nutritive apparatus, and to 

 those in which the young are provided with such an apparatus, it has a certain 

 convenience. 



