LHe, MOLLUOSCA 21 
= 
hectocotylus, is caducous and travels independently in search of 
the female. In those Molluscs which do not copulate the eggs 
are fertilised after oviposition. The eggs are laid separately in the 
Amphineura, in the more archaic Gastropoda, in the Scaphopoda, and 
in almost all the Lamellibranchia (that is to say, generally, in the 
forms that do not copulate), but in the majority of aquatic Gastro- 
pods and in the Cephalopods the eggs when laid are united into 
a gelatinous or coriaceous nidus, which may be attached (benthos) 
or floating (plankton). As a rule, Molluscs do not nurture their 
progeny, and when once the eggs are laid they take no further 
heed of them. Some of them, however, retain their eggs till the 
time of hatching, and are therefore called incubatory forms (Fig. 8). 
Examples of incubatory forms occur among the Lamellibranchs, 
especially the specialised eulamellibranchiate Submytilacea ; among 
marine Gastropods (/ermetus, etc.), among freshwater Gastropods 
(Melania, etc.), and even among the octopodous Cephalopods (47rgo- 
nauta), but the number of truly 
viviparous forms is very small. 
Callistochiton among the Amphi- 
neura and several genera of 
aquatic and pulmonate Gastro- Sah 
pods are the only instances. Fie. 8. 
The number of eggs laid is very “gy amit et ide iow, with ou 
variable. It is always greater in 
the case of those marine Molluses which abandon their eggs to 
the mercy of the waves than in those which deposit them in a 
nidus, agglomerated together in ribands or in shells in which the 
embryos are naturally protected. Thus Ostraca may lay as many as 
60,000,000 eggs, Chiton 200,000. On the other hand, numerous 
eggs are found in the nidus of certain Nudibranchs (50,000 in the 
case of Doris), Cephalopods (Loligo, 40,000), and pelagic Gastropods 
(Cymbulia, 1200). In all cases in which numerous eggs are laid 
free larval forms are developed, but when the whole of the develop- 
mental stages are passed through within the egg-membranes, and 
when the young individual is hatched with the characters of its 
parents and undergoes no metamorphoses, the number of eggs is 
generally small (Cenia, 4-12), or the greater number of the eggs laid 
is absorbed and furnishes nutriment for a few embryos (Buccinum, 
Purpura, etc.). In the following cases also eggs are laid in small 
numbers :—(1) In incubatory forms, such as Vermetus, where from 
120-240 eggs are incubated in the pallial cavity, under the 
protection of the shell; (2) in terrestrial and fluviatile species, in 
which the number of eggs is always smaller than in marine forms. 
In this case caenogenesis or embryonic condensation is the rule ; 
the young animal quits the egg in the adult form, and there is no 
need for a large number of embryos. Instances in point are— 
