108 ' METAZOA IN GENERAL 



aquatic forms, like rotifers and water fleas, which are also eaten in great 

 numbers by fish and other larger aquatic animals. 



If rapidity of multiplication is the end reached in parthenogenesis, 

 this is attained to a still greater degree if the animal does not wait to 

 become mature before it becomes capable of reproducing. Reproduc- 

 tion by an immature animal is known as pedogenesis and occurs in a 

 number of insects; for instance, the larvae of certain gall gnats, the pupae 

 of some midges which produce eggs capable of developing without 

 fertilization, and certain animals having biparental reproduction, as is 

 shown by the production of young by immature salamanders, known as 

 axolotls. 



132. Types of Fertilization. — Animals which are monecious are 

 capable of fertilizing their own egg cells, though actually in nature few 

 such animals do so. When this occurs, the phenomenon is known as 

 self-fertilization. In the case of animals which are diecious the fertiliza- 

 tion of the egg cell of one individual must be by the sperm cell of another. 

 This is known as cross-fertilization. Cross-fertilization is not the same 

 as hybridization, the latter term being applied usually when the two indi- 

 viduals belong to different species, varieties, races or strains. Cross- 

 fertilization is a very general phenomenon and is practically universal 

 among the higher animals; hybridization is much less frequent. 



133. Oviparity and Viviparity. — Many animals retain for a time 

 within their bodies their egg cells and the embryos which develop from 

 them and give birth to living young. Such animals are termed vivi- 

 parous and the phenomenon is viviparity. On the other hand, a great 

 many pass the egg cell out of the body for development. These forms 

 are termed oviparous and the phenomenon oviparity. In oviparous 

 animals the egg cell when passed out of the body is usually provided with 

 a greater or less number of protective envelopes various in character, and 

 to the egg cell plus all of these envelopes is applied the term egg. In 

 some cases fertilization takes place within the body before these enve- 

 lopes are added, and here, as in viviparous animals, the phenomenon is 

 referred to as internal fertilization. On the other hand, the egg may be of 

 a character which permits fertilization after passage from the body. 

 Such a type of fertilization is termed external fertilization. 



134. Metagenesis. — There are animals in which both sexual and 

 asexual types of reproduction occur, and these in alternate generations. 

 One or more generations produced in one manner are followed by one or 

 more produced in the other. This phenomenon is termed metagenesis, 

 or alternation of generations. It is illustrated best among some marine 

 hydroids and jelly fishes, in connection A\ith the study of which it will 

 be more fully described. 



