130 THE GASTROPODA 



unfertilised ova are generally laid one by one and are not united 

 by an accessory envelope (Patellidae, Haliotis, certain Trochidae of 

 the sub-genera Gibbula and Trochocochlea), but in Fissurella and in 

 Trochidae of the sub -genus Zizyphinus they are united by a 

 gelatinous investment. In the species that copulate the ova are 

 deposited within a few days after the act of copulation, the time 

 varying from one day in sundry Nudibranchs to as many as fifteen 

 days in some species of Helix. The nidus may assume very various 

 shapes. In aquatic species the shells surrounding the eggs may 

 be embedded and united in a single gelatinous mass, which may be 

 ribbon-shaped, more or less coriaceous, attached in littoral species, 

 Hoating in pelagic species. This is more especially the case in the 

 Euthyneura viz. in the Basommatophora and Opisthobranchia, in 

 which the ribbon is often coiled into a spiral and also in many 

 Taenioglossa (Littorinidae, Bissoidae, Hydrobiidae, etc.). In such 

 cases each egg-shell contains a single ovum. Again, the egg-shells 

 may be coriaceous (Rachiglossa), more or less independent, simply 

 attached to one another (uccinuin, Fusus, Pyrula), or fixed side by 

 side on a common support (Purpitra, Murex, Nassa, Fig. 108, 

 Trophon, Valuta, etc.). In this case each shell contains a consider- 

 able number of ova, but all of them do not complete their develop- 

 ment. As special forms of nidus may be mentioned that of 

 Natica, in which the eggs are united by agglutinated sand into a 

 horny-looking ribbon coiled in a ring ; that of Lamellaria, whose 

 eggs are deposited in >a sort of nest excavated in colonies of 

 Synascidians. Finally, various forms of Streptoneura attach their 

 eggs to various parts of their bodies, and 

 thus appear to be more or less incubatory, 

 as, for instance, Hipponycidae and Capu- 

 lidae (in Calyptraea the eggs are attached 

 below the neck) ; or they may attach them 

 to the external surface of the shell, e.g. 

 Neritina, HydroUa ulvae, and in excep- 

 tional cases Bissoa ; or to the internal face 

 of the shell, e.g. Vermetus (Fig. 45, ov). 

 In the oviparous Janthinae the eggs are 

 attached to the float (Fig. 135, I). 



The stylommatophorous or terrestrial 

 Pulmonates generally lay in the earth 



FIG 10S 



isolated ova enclosed either in a gelatinous 



Egg-capsule of Naxsa retimdata, , , T . , x . , .,, , 



x 12. o, aperture ; ov, eggs. envelope (Lvmax, etc.) or in a calcined 

 shell, e.g. certain species of Helix, Testacella, 



etc. In Bulimus these eggs may attain a length of three centi- 

 metres, thus exceeding in size the eggs of many birds. The eggs 

 of Ampullaria have also a calcified envelope ; they are laid in 

 the water and are agglomerated together. When Succinea lays its 



