HY 11. LEIGIITON KESTEVEX. 447 



The embryonic stage closes with the hirva, which is the phyl- 

 embryo. The pupa has acquired the adult organs and must 

 therefore be regarded as the neanic stage, since it is that in which 

 the adult characters first appear. The nepionic stage must con- 

 sequently be, in a silkworm for example, that during which it 

 spins its cocoon and becomes metamorphosed into the pupa. The 

 imago is the ephebic stage. 



Homologising the moUuscan stages with the above, the veliger 

 is the phylembryo. The nepionic stage is so contracted as to be 

 generally unrecognisable and (probably) non-existent conchyla- 

 ceously. It is however, here as in the Le'pidoptera^ that stage 

 during which the larval organs degenerate and disappear. The 

 neanic stage during which the adult characters appear and undergo 

 development is generally so graduated into the succeeding ephebic 

 stage that only in a few instances can its ultimate limit be 

 defined. Early adult shell-structure is, however, doubtless 

 neanic, as is also the pseudoprotoconch. 



Three types of transition from eynhryonic to neanic sheU- 

 structure, and suggestions as to their explanation a7id significance. 

 — The investigations of the writers mentioned in the appended 

 bibliography have brought to light three very distinct types of 

 transition from embryonic to neanic shell-structure, viz. : — 



(1) Those in which the embryonic is faintly, if at all, defined 

 from subsequent structure. Examples are Melo indicus, Lotoriuni 

 abbotti, Tenison- Woods, and most species of Tripho7'a. 



(2) Those in which there is an abrupt change from embryonic 

 to subsequent structure. This occurs in all the recent species of 

 Lotorium. 



(3) Those in which a varix is thrown up at the conclusion of 

 the protoconch before the neanic structure is initiated. Accord- 

 ing to F. C. Baker* most of the Murices fall into this category. 



Remembering that the primitive shell-gland is distinct from 

 the area which secretes the adult or, as Lankester'f aptly terms 



* Baker, Proc. Acad. Nat. Sci. Philad., 1890, p. 66 et seq. 

 t Lankester, Ency. Brit. edit. 9, xvi., p. 639, ISSo. 



