106 GENERAL EMBRYOLOGY 



after a complicated "courtship" (Urodeles, Jordan), the sperm are 

 received by the female in or near the reproductive cavities or openings. 

 Usually in such instances the sperm are not scattered freely, but are 

 contained within definite packets or cases called spermatophores (Fig. 

 53). In the Urodeles these are simple masses of spermatozoa enclosed 

 in a thin envelope; they are discharged by the male and then picked up 

 by the cloacal margins of the female and stored until the eggs are ready 

 to be fertilized. In the amplexus of the earthworm and many Gastero- 

 pods, there is a mutual exchange of spermatozoa between two herma- 

 phroditic individuals, the sperm being received into storage cavities 

 and retained until the eggs are deposited. Such a receipt of sperm or 

 sperm packets into storage cavities is quite common, and the sperm 

 may in these cases remain alive for long periods, even for three or four 

 years (honey bee, some snails). The spermatophores are sometimes 

 very elaborate affairs containing a complex mechanism arranged so as 

 to discharge the sperm just at the time the female is depositing the eggs 

 (Fig. 53, C). Among the more complex are those formed by the male 



FIG. 54. The Trematode, Bilharzia hcematobia. Two individuals living in 

 copula. After Fritsch. X 14. o, ova in oviduct. d\ male; $, female. 



squid (Loligo) and transferred to the buccal membrane of the female, 

 where they remain attached pending the time of spawning (Drew). 



Among forms where internal fertilization is the rule, this is more 

 frequently the result of a definite act of copulation, by which the sperm 

 are transferred directly to a reproductive cavity of the female through a 

 male intromittent organ. This occurs in most Insects, many Turbellaria, 

 Crustacea, Molluscs, and in most of the higher Vertebrates. This gen- 

 eral relation, carried to an extreme may result in symbiosis, or even in 

 the parasitic character of the male upon or within the body of the female. 

 Thus in some of the Cirripedia, degenerate " complemental " males are 

 found living semiparasitically within the body of the female. Several 

 of the Trematoda live in pairs within a single cyst. In Bilharzia (Tre- 

 matoda) the female lives permanently in a groove on the body of the 



