BY H. L. KESTEVEN. 51 



in the sense that Jackson proposed them (23). In some cases 

 the primitive shell-gland is never functional. 



Instead of V tloconch, the term Nectoconch would have 

 been more applicable. I had failed to realise, when the 

 term was proposed, how constantly the thimble-shaped sym- 

 metrical nucleus is formed by the Trochosphere . I now sub- 

 stitute the term Xecfoconch, and define it as "that portion 

 of the protoconch which is formed during the pelagic exis- 

 tence of the embryo, or the equivalent stages in the case of 

 species such as all Puli)ioiiat(i, which have no pelagic exis- 

 tence." Or more precisely "that portion of the protoconch 

 formed after the flattening out of the primitive shell-gland, 

 and before the loss of the velum." In the case of most, if 

 not all, i\ udihra/ic/is this is the only shell formed. 



Before the Nepioconch can be discussed, it is necessary to 

 define the Xepionir stage in the Gastropoda. 



Throughout the whole animal kingdom, it is possible to 

 recognise embryonic and postembryonic divisions of the life- 

 history. In the great majority of cases, we can also recognise 

 juvenile (Xea/iic) and adult (Ep/iehrir) stages in the post- 

 embryonic division, and in many instances the acquisition of 

 new or loss of established characters defines a stage of Old Age 

 (Gero/ifir). The occurrence of sexually mature larvae (Axo- 

 lotl, Aphidie, etc.), in no way lessens the value of the gener- 

 alisation, but serves only to point the necessity of care in its 

 particularisation. 



Again, in the vast majority of cases we can recognise a 

 typical larval stage, and one or more pre-laval stages [Jack- 

 son recognises four (23)] in the Embryonic division of the 

 ontogeny. And here again, the occasional apparent or real 

 suppression of one or more of these stages emphasises the 

 need for care in the application of any nomenclature arising 

 out of the generalisation. 



In all those cases where larval organs are developed, a 

 period of metamorphosis will intervene between the larval 



