STRUCTURE AND FUNCTION IN INVERTEBRATES 



In pseudocoelomate animals the gonads lie in the pseudocoel and connect 

 directly with reproductive ducts. Ova are fertilized internally, and zygotes 

 are usually encased in protective shells before being passed to the outside. 

 In many pseudocoelomates, however, the eggs are retained until they hatch in 

 specialized portions of the female reproductive tract, and the offspring emerge 

 as juveniles. Many of the Nematoda and all the Acanthocephala are endo- 

 parasitic forms with especially well-developed reproductive systems. 



It will be recalled that one of the distinguishing characteristics of the 

 true coelom is its consistent relationship to the reproductive system; this 

 relationship can be demonstrated or inferred in all eucoelomate animals. In 

 many of the more primitive annelids, for example, there are no permanent 

 gonads or reproductive ducts. The peritoneal lining of the coelomic cavities 

 is essentially a germinal epithelium, in which gametes are periodically pro- 

 duced and released into the coelom; the gametes reach the exterior by rupture 

 of the body wall or by passage through the nephridial tubules. Sometimes 

 special gonoducts have been developed to conduct gametes to the outside. 

 At this primitive level copulation does not occur; male and female gametes 

 are simply shed into the surrounding medium for external fertilization and 

 development. The more highly evolved oligochaete annelids have permanent 

 gonads and elaborate accessory chambers and ducts, which are clearly 

 localizations and specializations of primitive coelomic structures and 

 functions. The oligochaetes are almost entirely limited to fresh-water and 

 terrestrial habitats, and the reproductive specializations, including copula- 

 tion and the production of cocoons in which fertilization and development 

 occur, may be interpreted as adaptations to environmental conditions. 



Among mollusks and arthropods, in which the coelom is not well developed 

 in the adult animal, the cavities within the gonads may be interpreted as 

 coelomic spaces, lined by germinal epithelium, in which the gametes are 

 produced. The reproductive ducts are gonoducts, retaining their primitive 

 connections with the coelomic cavities. Among bivalve mollusks reproductive 

 mechanisms are simple; fertilization often occurs within a part of the mantle 

 cavity of the female, and early developmental stages may be passed within 

 the "brood pouches" in the gills. In gastropods and cephalopods reproduc- 

 tive systems are much more complex. Here some kind of copulation is the 

 rule; fertilization is internal, and the female reproductive system possesses 

 elaborate glandular portions which secrete shells and egg cases about the 

 zygotes. Although many snails are hermaphroditic, cephalopods are typically 

 dioecious. Among Arthropoda reproductive mechanisms vary from relatively 

 simple to highly complex; but even among the simple forms there is usually 

 some kind of copulatory behavior, in which spermatozoa are transferred into 

 a seminal receptacle of the female. In many crustaceans, such as the cray- 

 fish, this is an externally opening, sac-like pocket having no direct con- 

 nection with the female reproductive system, and ova are fertilized externally 

 as they pass the opening of the receptacle. In terrestrial forms, however, 

 as in insects, the seminal receptacle is a diverticulum of the common oviduct, 



533 



