66 THE CONSTITUTION OF THE GASTROPOD PROTOCoNCH, 



In case No. 1, the Nematode, there can be no doubt that 

 the different environments produce changes in the reproduc- 

 tive organs of the maternal organism, which, in turn, affect 

 the life-history of the embryo, and the same may be said uf 

 case No. 2, Fakemonetes. In the case of Alpheus, the proof 

 IS wanting, but there can be little doubt that here also the 

 variation in the ontogeny is due to environmental conditions 

 acting through the maternal organism. 



One form of the general question stated above is : — What 

 explanation, other than phylogenetic, if any, may be offered 

 for the fact that the protoconchs of nearly-related species pre- 

 sent variations in the size of the caliculus* with resultant 

 varying asymmetry and symmetry of initial coiling ? 



The caliculus of the N ectoconch was commenced by the 

 Trochosphere before the asymmetrical growth of the larva 

 began, and gives place to a coiled shell in the Trochosphere 

 or early Veiiyer stage when the shell-secreting area partakes 

 of the general asymmetry. Since the asymmetry of the larva 

 appears very early, it follows that the caliculus is, in the 

 majority of cases, formed before the larva is so far advanced 

 as to take food, even where the whole larval development is 

 intra-capsular, and the larva accordingly floating in a nutri 

 ent medium. Hence, the size of the caliculus is determined 

 by the size of the egg; if that be large, the Trochosphere and 

 early Veliger will be large, and so also the shell formed 

 thereby. 



If the plan of coiling be open and discoid, as, for instance, 

 in Spirilla, a large caliculus will not cause asymmetry thereof, 

 but if, as in the example quoted, the larva fails to increase 

 in size as rapidly as when secreting the caliculus, then that 



•This term is introduced here for convenience and precision of descrip- 

 tion and argument. It is applied to the bilaterally symmetrical thimble- 

 or saucer-shaped nucleus which forms the starting point of all protoconchs. 

 It differs in character from the conchogeiietic terms proposed earlier inas- 

 much as it applies to a particular portion of tiie protoconch formed at any 

 stage of development, not to any portion formed during a particular stage. 



