GENERAL ZOOLOGY 



and fertilization is internal. Most insects deposit shelled eggs, well protected 

 against damage or desiccation. In some of the highly evolved Diptera, the 

 zygotes hatch within the body of the female, who then deposits active, first- 

 stage larvae on or near sources of food. In other specialized dipterans, even 

 the larval stages are passed within the reproductive system of the mother, 

 and the oflfspring appear as fully mature larvae, ready to pupate. In at least 

 one group of insects with parasitic larvae, early larval development is marked 

 by polyembryonic reproduction reminiscent of that noted in larval trematodes. 



Among Echinodermata, reproductive systems are comparatively simple. 

 The gonads are sac-like or branching, tubular structures, and the reproduc- 

 tive ducts lead directly from the gonads to the exterior. In the adult there 

 is no clear relationship between the gonads and the coelom, but in embryonic 

 stages the genital rudiment develops in connection with other derivatives of 

 the coelom. Reproduction in echinoderms never involves copulation or in- 

 ternal fertilization, but there are many instances in which embryonic develop- 

 ment occurs within superficial brood pouches on the body of the female. 



There are two special phenomena of reproduction that are of surprisingly 

 general occurrence among invertebrates, although their distribution has no 

 demonstrable relationship to phylogenetic position or affinities. These two 

 phenomena are hermaphroditism, or monoeciousness, and parthenogenesis. In 

 monoecious animals each individual of the species develops complete and 

 functional reproductive systems of both male and female sexes. It is excep- 

 tional, however, for fertilization to involve spermatozoa and ova from the 

 same individual. Most commonly, there appear to be special safeguards, 

 either morphological, functional, or chronological, which prevent self- 

 fertilization. In the earthworm, for example, the structural relationships 

 are such that it is practically impossible for self-fertilization to occur. 

 In some planarians it has been demonstrated that spermatozoa do not become 

 active in the presence of ova from the same individual; and in many other 

 monoecious animals the testes and ovaries do not become functional at the 

 same time. Granting that such close inbreeding as self-fertilization would 

 probably be detrimental, we are still faced with the problem of understanding 

 why animals should be bisexual and yet be prevented from producing oflf- 

 spring in isolation, a process to which monoeciousness would seem primarily 

 adapted. One provisional answer suggests itself. Usually a population con- 

 sists of males and females in approximately equal numbers. This means that 

 only half the random encounters between two individuals can be reproduc- 

 tively fruitful, since a given male will be as likely to encounter another 

 male as to meet a female. If every individual is both male and female, how- 

 ever, then every encounter is doubly effective; insemination is mutual, and the 

 eggs of both individuals can be fertilized as a result of a single meeting. 

 It is perhaps significant that monoeciousness appears to occur most commonly 

 among animals which are slow-moving and relatively inactive, or which in- 

 habit environments that do not facilitate or permit frequent encounters 



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