Life of the Snails 29 



eggs may develop in an undisturbed, food-laden medium. Very frequently 

 extra eggs (nurse eggs) are added which serve as food for the young that 

 hatch first. The young may emerge from the egg cases as miniature replicas 

 of their parents and commence a life of crawling and feeding, or they may 

 escape as free-swimming larval forms. The latter are known as veligers and 

 possess special organs for swimming. The larval shell is often quite different 

 from the adult shell and, in some species, there may be an extra shell or 

 echinospira encasing the entire veliger. 



There are many types of tgg cases, and some of these are illustrated in 

 figures 8 and 9; others are briefly described under the generic or family 

 discussions in the identification section. Several types of egg-laying may be 

 found within a single family or even genus. 



1. Eggs Laid in Capsules and Attached to the Bottom: 



Rissoidae, Caecum, Epitomum, Thais, Miirex, Coins, Neptunea, Busy- 

 con, Buccinum, MeloTigena, Nassarius, Bela, Mangelia, Voluta, Conus, Co- 

 lumbella, Fusinus, Cancellaria, Marginella, Neritidae and others. Of these, 

 some have nonpelagic development: some Murex, Cojjus, Natica and most 

 Marginella; others have pelagic, free-swimming young: Nerita, some 

 Murex, some Conus and some Natica. 



2. Eggs Laid in Gelatinous Masses or Strings: 



Acmaea, Gibbula, Fissurella, Lacuna, Littorijia obtusata, some Turri- 

 tella, Bittium, Triphora, Cerithium, Capulus, Strombus, Aporrhais, Cassis, 

 all opisthobranchs and heteropods. 



3. Eggs Laid in Capsules and Protected by the Female: 



Crepidula, Calyptraea, Janthina, Cypraea, Hipponix, Vermetus. 



4. Eggs Laid in Sandy Collars: 



Folinices and Natica. 



5. Eggs Shed and Developing Suspended in Water: 



Some Acn/aea, some Gibbida, Tectarius, some Littorina, Haliotis, and 

 the heteropods, Atlanta and Oxy gyrus. 



In some groups of snails which are more or less sedentary, the tgg cap- 

 sules may be protected by the female. In the cap-shell, Hippojiix, the under- 

 side of the foot of the female has a tough, reinforced ridge of flesh to which 

 she attaches her gelatinous tgg sacs. In some worm-shells, Verv^etus, whose 

 shells are permanently attached to the rocks, the eggs are deposited on the 

 inside of the female's own shell. 



The time and length of breeding differs among mollusks depending 

 mainly on the geographical locality, the temperature of the water, phases of 

 the moon and the inherent characteristics of the species. Some species spawn 

 once a year for a few weeks only, while others may produce eggs half of 

 the year as long as the temperature is suitable. 



The eggs, larvae and young have been described for many species by 



