GENERAL ZOOLOGY 



mental resistance. The reproductive processes developed among the inverte- 

 brates are almost infinitely varied, but all of them are understandable as 

 means to these ends. 



Mechanisms of reproduction in many of the simpler animals do not involve 

 the development of organs or of systems, being of an asexual nature. Asex- 

 ual reproduction is common among the protozoans and the more primitive 

 metazoans. Rapid increase in protozoan populations occurs by the fission, 

 or division, of one mature animal to produce two daughter individuals 

 approximately equal in size. The organelles of the mature animal, as among 

 ciliates, are commonly divided between the daughters, each of which then 

 elaborates the parts which it lacks. Fission in a protozoan requires, of course, 

 but one cell division to produce two individuals where there was originally 

 one. Reproduction by fission also occurs among many of the simpler 

 Metazoa, as in planarians and other turbellarians. Here, however, the 

 activities of many cells are involved in the replacement of missing parts 

 and in the regulation of form in the daughter individuals. Other types 

 of asexual reproduction found in metazoans are budding, common among 

 coelenterates, and a somewhat similar process occurring in the pro- 

 liferative larval stages of digenetic trematodes. In budding, a daughter 

 individual is not produced "all at once," as it were, by division of the 

 parent body, but develops relatively slowly, growing at the expense of the 

 parent until mature enough to become independent. Reproduction in the 

 larval stages of trematodes involves the subdivision of internal masses 

 of cells into smaller clumps, each of which then develops into a complete 

 individual of the succeeding stage. This process might be termed internal 

 budding; it is sometimes considered as a process of polyembryony (see 

 p. 209), but in trematodes larval reproduction does not immediately follow 

 the production of a zygote. Wherever asexual reproduction occurs among 

 metazoans, it is generally correlated with a marked capacity for regenera- 

 tive growth. Both the replacement of parts lost through injurv and the 

 development of missing parts in a daughter individual are processes involving 

 the activities of undifTereptiated, totipotent cells. 



Although asexual reproduction is an efifective means of rapidly increasing 

 the population, species with exclusively asexual reproduction are rare among 

 animals. The more primitive groups, including Protozoa, have developed 

 methods of sexual reproduction in addition to asexual processes, and among 

 most higher invertebrates reproduction is exclusively sexual. New individuals, 

 beginning as zygotes, are formed by the union of male and female gametes 

 produced usually by difTerent parents; thus the new individual may receive a 

 mixture of the hereditary characteristics of the two parents, and new genetic 

 combinations may be continually produced in the population. The various 

 systems of organs, chambers, and ducts which form the reproductive systems 

 of most metazoans are elaborations of originally simple arrangements for the 

 transport of gametes from the gonads to the exterior. In many invertebrates 

 specializations of the male reproductive system have developed to facilitate 



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