GENERAL ZOOLOGY 



Coelom 



Body wall 



/ ^ Septum 



Mouth 

 Ovary or testis Oviduct or male duct Ovary or testis Nephridium 



Anus 



Coelom 



Testis Male duct 



Ovary Oviduct 



Male system 



Seminal 

 receptacle 



Female system 



Ovipositor 



Fig. 17.10. Reproductive mechanisms in some of the more complex invertebrates; schematic. 

 ,1, e;ametes mature in individual sac-like gonads and are discharged to the outside, where 

 fertilization and development occur (Nemertinea). B, gametes are produced bv proliferation 

 of the coelomic lining, or in special chambers derived from the coelom; gametes reach the 

 outside through nephridia or coelomoducts, and fertilization is external (Annelida). C, 

 ovaries and testes are not obviouslv related to the coelom; special structures, such as seminal 

 vesicles, seminal receptacle, penis, and ovipositor, develop in relation to internal fertilization 

 (Insecta). 



animals become more complex. In fact, the simplest bilateral animals, the 

 Platyhelminthes, have surprisingly complicated reproductive systems. Be- 

 ginning with Acoela, whose only organ systems are reproductive systems, 

 turbellarians develop very effective reproductive mechanisms. These include 

 provisions for copulation, internal fertilization, and the deposition of 

 shelled capsules in which the young pass their embryonic stages. Each in- 

 dividual turbellarian commonly possesses both male and female systems, al- 

 though cross-fertilization appears to be the rule. Thus, the extremely 

 well-developed reproductive systems of the parasitic trematodes and cestodes 

 are foreshadowed in their free-living relatives, and the adaptations for 

 parasitic life involve chiefly differences in degree, not in fundamental 

 organization. In contrast to the Platyhelminthes, the Nemertinea, of the 

 same general grade of organization, have very simple reproductive organs, 

 consisting of series of pouch-like ovaries or testes, each with its individual 

 external opening. 



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