14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 



EVOLUTION OF THE GENITAL EXIT APPARATUS 



The free liberation of the gametes into the surrounding medium, 

 where conjugation takes place as circumstances permit, is the usual 

 mode of propagation with aquatic animals from coelenterates to 

 fishes, though there are various exceptions to the rule. This method 

 of propagation, however, becomes entirely impracticable for terres- 

 trial animals. Hence, animals that live customarily on land either 

 revert to the water at the breeding season, or they develop a genital 

 apparatus for the direct transfer of the spermatozoa from the male 

 into the body of the female. Propagation by sex mating, however, 

 demands a fixed location of the genital orifices at definite points on 

 the body wall in each sex, and the presence of an efifective ejaculatory 

 apparatus in the male. It is usually increased in efficiency by the 

 development of a male intromittent organ, a female receptaculum 

 seminis, and various devices for copulation. 



All modern Arthropoda, except parthenogenetic and hermaphroditic 

 forms, propagate by sex mating. This fact need not be construed to 

 mean that the ancestors of the arthropods were terrestrial, but, if the 

 mating habit was established by aquatic progenitors, it made the 

 arthropods easily adaptable to life on land. Among aquatic forms, 

 however, the sperm are not in all cases inserted into the genital ducts 

 or a receptaculum of the female. With the Pycnogonida, the Xipho- 

 surida, and many of the Crustacea, though copulation takes place, the 

 eggs are inseminated outside the body of the female at the time of 

 mating, and are then carrjed by one sex or the other, or deposited at 

 the bottom of the water. In some of the Crustacea packets of sperm 

 are attached to the under surface of the female's body, and fertiliza- 

 tion of the eggs takes place later. With the majority of the arthropods, 

 however, the spermatozoa or spermatophores are received into an 

 ectodermal pocket of the female (thelecum, receptaculum seminis, 

 spermatheca) situated near the openings of the oviducts, usually in 

 close proximity to them. The eggs in most such cases are then ferti- 

 lized as they issue from the ducts. The storage of the sperm in an 

 ectodermal receptacle is thus only a modification of external insemina- 

 tion. In some arthropods, however, the sperm are introduced directly 

 into the oviducts, and fertilization of the eggs may then take place in 

 the ducts or in the ovaries. Finally, there is often present in the 

 female an ectodermal pouch, the bursa genitalis, or genital chamber 

 (bursa copulatrix), which receives the end of the male intromittent 

 organ, and contains the spermathecal and oviducal apertures. 



The primary exit ducts of the gonads must be principally meso- 

 dermal structures, considering their coelomic origin, but if they have 



