STRUCTURE AND FUNCTION IN INVERTEBRATES 



Osculum 



^ o,"*^ , Choanocyte 



Excurrent Spongocoel / j^ 



canal \ V / 



Incurrent 

 canal 



- Epidermis J\ 

 -Gastrodermis / | 

 ;-Enteron Testis 

 i>Ova 



- Gastrodermis 



Incurrent External Ovum 

 canal surface . 



Fig. 17.9. Reproductive mechanisms in some of the simplest invertebrates; schematic. A, 

 gametes arise from cells of the mesenchyme-like middle region. Ova are fertilized in place 

 and develop into flagellated larvae, which break through the choanocyte layer into the ex- 

 current canals and are discharged via the spongocoel and osculum (Porifera). B, gametes 

 arise from cells of the gastrodermis, are discharged into the coelenteron, and pass to the out- 

 side through the mouth-anus (Scyphomedusae and Anthozoa). C, gametes arise from 

 interstitial cells of the epidermis and are discharged into the surrounding water (many 

 Hydrozoa). 



the introduction of spermatozoa into the body of the female at copulation. 

 In the female special mechanisms are often provided for the storage of sperm 

 received at copulation, for the internal fertilization of ova, for the deposition 

 of protective shells about the resulting zygotes, and in many instances for the 

 retention of zygotes during all or part of the embryonic period. Within any 

 single phylum these reproductive structures are generally homologous in all 

 classes; but when comparisons are made between animals in different phyla, 

 it becomes evident that structures of very diverse origins have become 

 adapted to similar functions in different groups (Figs. 17.9, 17.10). 



In keeping with their primitive levels of organization, Mesozoa and 

 Porifera have no reproductive organs or systems, but only reproductive cells 

 which differentiate into gametes. In Coelenterata and Ctenophora reproduc- 

 tive cells become localized in specific, temporary aggregations of cells which 

 are termed "ovaries" and "testes," but these organs are of a very primitive 

 nature, and there are no true reproductive systems. In all these simple 

 groups, with few exceptions, gametes are released into the surrounding water, 

 fertilization is external, and there are few if any provisions for the care and 

 protection of the developing young. 



Reproductive systems are of general occurrence among the Bilateria, but 

 it cannot be said without exception that these increase in complexity as 



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