THE MOLLUSC A 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 (Vermetus, etc.), among freshwater Gastropods 

 (Melania, etc.), and even among the octopodous Cephalopods (Argo- 

 iiauta), but the number of truly 

 viviparous forms is very small. 

 Callistochiton among the Amphi- 

 neura and several genera of 

 aquatic and pulmonate Gastro- 

 pods are the only instances. Fia 8 - 



TliP rmmhpr nf PO-O-<S lairl i vprv Stcnogjira mamillata, left side view, with four 

 t eggS laid IS Very embryos in the oviduct, em, embryo. 



variable. It is always greater in 



the case of those marine Molluscs 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 Ostraea 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 (Litliffo, 40,000), and pelagic Gastropods 

 (Cymbul-ia, 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 Vermetux, 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 



