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 //eliz. 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, 
floating 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, Rissoidae, 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 (Luccinwn, Fusus, Pyrula), or fixed side by 
side on a common support (Purpura, Murex, Nassa, Fig. 108, 
Trophon, Voluta, 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 
vo thus appear to -be more or less incubatory, 
as, for instance, Hipponycidae and Capu- 
lidae (in Calyptraca the eggs are attached 
below the neck); or they may attach them 
to the external surface of the shell, e.g. 
Neritina, Hydrobia ulvae, and in excep- 
tional cases issoa; or to the internal face 
of the shell, e.g. Vermetus (Fig. 45, 00). 
In the oviparous Janthinae the eggs are 
attached to the float (Fig. 135, 0). 
The stylommatophorous or terrestrial 
Pulmonates generally lay in the earth 
Fic. 108. < : : 
isolated ova enclosed either in a gelatinous 
Egg-capsule of Nassa reticulata, a : : 
x 12. 0, aperture ; ov, eggs. envelope (Limaz, etc.) or in a calcified 
shell, ¢.g. certain species of Helix, Testacella, 
ete. 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 
