58 THK CONSTITUTION OP THE GASTROPOD PROTOCONCH, 



laid stress upon the importance of change in environment or 

 functional activity in producing those differences, which 

 enable one to define the constituents of the Frotoconch, The 

 change in environment probably acts to bring about the 

 change in functional activity which, it is conceivable, was 

 potentially present, and only in abeyance in the absence of 

 the necessary environment. Thus it is possible that, in the 

 species referred to above, the power to secrete a calcareous 

 shell was already present in the free-swimming Velujer with 

 a horny shell, but was in abeyance owing to the absence of 

 calcium salts in a form which might be assimilated. Within 

 the egg-capsule, the only factors which will vary the form of 

 the shell are changes in the form and functional activity of 

 the mantle or other shell-secreting surface. Changes in the 

 mantle may be abrupt and productive of sudden change in 

 tne shell-characters, or may be gradual, the shell changing 

 jjari 2}a.'isu. The abortion of the velum, ^f/- se, need not be 

 productive of any shell-characters ; it is not accompanied by 

 any change in environmental conditions, such as would pos- 

 sibly be reflected in functional changes. If, however, all the 

 Gastropoda be evolved or descended from forms with pelagic 

 larvae, and the universal occurrence of Trorhosphere and 

 V eVu/cr causes this to be generally conceded : then it is con- 

 ceivable that, even in those forms which now pass those stages 

 within the egg-capsule, the abortion of the velum is marked 

 oy a pause in growth, and occurs at the same time as the 

 acquisition of adult characters by the mantle, since such is 

 the case in nearly all species with pelagic larvae at present. 



The conclusions of this section may be summarised as 

 follows : 



In the ontogeny of the great majority of the membiers of 

 the animal kingdom there is a period when organs character- 

 istic of a larval stage are aborted ; and since, in many cases, 

 this period is also marked by other important morphological 

 and physiological changes, it is advisable to designate this 

 period always by the same name. The term Nepiotiic pro- 



