28 American Seashells 



males usually occupy positions in the vicinity of the female and move to the 

 mating position at night. Occasionally bachelors are found which either by 

 chance or choice remain solitary throughout the entire male phase. 



Soon after hatching from the tgg, and in one species (Crepidula adunca 

 Sowerby from Panama) even before hatching, a slender copulatory organ, 

 the verge or phallus, grows out from the body behind the right tentacle (fig. 

 7), As the female phase develops later in life, the verge begins to shrink and 

 is finally absorbed as the female organs take form. Associated with these 

 changes is a marked alteration in behavior, whereby the wandering individ- 

 ual, which was so characteristically masculine when young, now becomes 

 strictly sedentary. She receives her mate, lays her eggs in capsules beneath 

 her foot and broods her young until they are prepared for their own inde- 

 pendence. 



In our Common Slipper Shell, Crepidula fornicata, those individuals 

 which live on muddy bottoms where there are no solid objects to which 

 they can attach themselves, frequently pile up in groups of six to twelve or 

 more. These groups continue from year to year, newly arrived young in the 

 male phase attaching themselves to the top of the pile as the old, female- 

 phase individuals die at the bottom. 



Most marine prosobranchs, however, are of separate sexes (dioecious or 

 unisexual). While some species in which the sex products of both sexes are 

 discharged freely into the water have no outward morphological features, 

 there are a great number of gastropods in which the male has an external 

 copulatory organ or verge. The shape and position of the verge are often 

 used in classifying families, genera or species. 



Depending upon the species, and sometimes the genus, the females take 

 care of their young in a variety of ways. In some there is no motherly 

 instinct, and the eggs are liberated directly into the water where they float 

 away on the chance of being fertilized by the free-swimming sperm from a 

 nearby male. (See fig. 9 with Tectarms and Littorina.) In other types the 

 eggs are fertilized and undergo development to the adult-like form in the 

 uterine portion of the oviduct. Others have developed a kangeroo-like pouch 

 in the tissues of their back where the young are allowed to develop to the 

 adult form. Once liberated, however, the young do not return to the pouch. 

 Viviparity or the giving birth to young alive (technically ovoviviparity) is 

 known in Planaxis, Littorina saxatilis and a number of fresh-water species in 

 several different families. 



The Egg Cases of Snails 



Among a large proportion of the marine gastropods, the females form 

 special egg cases or capsules into which the eggs are placed, and where the 



