THE EVOLUTION OF SEX. 



receptacles and secreting glands are also very frequently in associa- 

 tion with the male ducts, and there is a long list of curious modifi- 

 cations utilized in the process of copulation. Thus, male frogs have 

 their swollen thumbs, and gristly fishes their "claspers," which are 

 modified parts of the hind limbs, and are inserted into the cloaca of 

 the female. The common snails eject a limy dart {spiculum amoris), 

 which appears to be a preliminary excitant to copulation. 



So too, in the female sex, the termination of the duct may be 

 modified for reception of the male intromittent organ, and special 

 receptacles may be present for storing the spermatozoa. Where 

 a single fertilization occurs, as in the queen-bee, previous to a long- 

 continued egg-laying period, the importance of a storing organ is 

 obvious. As the female is usually more or less passive during copula- 

 tion, the adaptations for this purpose are less numerous than in the 

 males. It is interesting to notice, that among amphibians, where 

 the male often takes upon himself distinctly maternal duties, one 

 case is known where the female seems more active than the male 

 during copulation. 



V. Egg-laying Organs. — Cases where the ova simply pass 

 out into the water, or on to the land, are of course associated with 

 the absence of any special organs. In a great many animals, how- 

 ever, more care is taken, and auxiliary structures are present. One 

 of the simplest of useful developments is exhibited by glands, the 

 viscid secretion of which moors the ova, and keeps them from being 

 set wholly adrift. In insects, where it is specially important that 

 the eggs should be well concealed, or buried in conveniently nutri- 

 tive material, hints of the ancestral abdominal appendages remain 

 as ' ' ovipositors. ' ' Throughout the series a great variety of structures 

 occur in this connection. 



VI. Brooding and Young-feeding Organs. — From very 

 lowly animals onwards, structures are present which are utilized in 

 the protection of the young in their helpless stages. The repro- 

 ductive buds of some coelenterates become true nurseries; in one at 

 least of the marine worms {Spirorbis spirillum), a tentacle serves as 

 a brood-pouch; various adaptations, such as tents of spines, or cavities 

 in the skin, are utilized in echinoderms. The young shelter under 

 the hard cuticle, or among the appendages of crustaceans, in the 

 gills of bivalves, and a cuttlefish has been seen with the eggs in 

 its mouth. Among the higher animals, the brood-pouch of Appendi- 

 cularia (one of the very lowest Chordata), the pockets of not a few 

 fishes, the cavities on the back of the Surinam toad, the pouches 

 of marsupials, are only a few instances amid a crowd. Sometimes, 

 especially in fishes and amphibians, — for example, the seahorse, with 



