STRUCTURE AND FUNCTION IN INVERTEBRATES 



between different individuals. Consider, as examples, the planarian, the 

 snail, and the earthworm, and the helminth inhabiting the intestine of a 

 vertebrate. Interpreted from this point of view, monoeciousness as it has 

 developed among animals clearly confers certain selective advantages and is 

 of definite survival value to a species. 



Parthenogenesis is the process by which offspring develop from eggs that 

 have not been fertilized. It should be understood that although partheno- 

 genesis does not involve fusion of nuclear material from two individuals, it is 

 not interpreted as asexual reproduction; it requires the production of a 

 differentiated gamete from which the new individual arises. Parthenogenesis 

 occurs in several diverse groups of invertebrates, notably among rotifers, 

 nematodes, crustaceans, and insects. In many rotifers males are completely 

 unknown, and reproduction is entirely a matter of the development of young 

 females from unfertilized eggs produced by mature females. In other rotifers, 

 and in some Cladocera among crustaceans, environmental changes in some 

 way induce the periodic production of males, and syngamy then occurs. The 

 resulting zygotes are usually covered with thick shells and are capable of 

 withstanding unfavorable environmental condiiions during their long period 

 of incubation. Reference has been made to the interesting reproductive 

 habits of the honeybee, in which the queen receives at one copulation all the 

 spermatozoa she will use during her life to fertilize her eggs. The spermato- 

 zoa are stored in a seminal receptacle, and by opening or closing the duct 

 leading from this chamber the queen is capable of what has been termed 

 "facultative parthenogenesis." Most of her eggs are fertilized and so develop 

 into diploid females, either workers or young queens. By withholding sper- 

 matozoa, however, the queen can deposit unfertilized eggs which develop into 

 the haploid males called drones. Other insects, such as aphids, also have 

 parthenogenetic reproductive mechanisms. 



The diversity of structure and of functional adaptations in the reproductive 

 systems of invertebrates has been sufficiently illustrated by the foregoing 

 review. Obviously, there is little general similarity throughout the inverte- 

 brate groups, beyond the presence of ovaries, testes, and their respective 

 ducts, although within a given phylum certain fundamental plans can be 

 recognized. Further complications of the reproductive systems are related to 

 the reproductive habits and modes of development in the many types of in- 

 vertebrates, and adaptations to special environmental conditions can often be 

 inferred. When these systems are compared from the standpoint of the 

 functions of their parts, it is clear that similar functions have been assumed in 

 many instances by structures that are not homologous. This is an indication 

 that many different kinds of reproductive systems have arisen independently 

 in the evolution of invertebrates. We may then conclude that in their 

 evolution various kinds of animals have solved their common problems of 

 reproduction in ways that are functionally comparable, although effected by 

 different structures. 



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