TYPE MOLLUSC A. 



363 



should be kept in mind, especially in view of another idea as to the gene- 

 alogy of the MoUusca which has been advocated by certain authors. Ac- 

 cording to this theory they have sprung from Turbellarian-like ancestors, the 

 creeping surface of the worm having become more muscular, and so having 

 given rise to the foot of the Mollusk, the dorsal region of the body elevat- 

 ing into the visceral hump. The nervous system of the Amphineura, with 

 its ladder-like arrangement, might readily be deduced from the arrange- 

 ment found in the Platyhelminths, and thus many points on which the Tro- 

 chophore theory throws no light become intelligible. 



This theory concerns itself mainly with the adult forms, yet it is not 

 impossible that a reconciliation between it and the Trochophore theory may 

 be possible. It has already been pointed out that the Trochophore may 

 possibly be the representative of a Turbellarian larva, and the same idea 

 may be applied to the Veliger. In other words, it is possible that the 

 MoUusca may have been derived from the Turbellaria, and that the ances- 

 tral worms possessed in their life-history a larva which, independently of 

 the adults, underwent a series of modifications leading to the Veliger. The 

 Veliger would then be the descendant of a Turbellarian larva, while the 

 adult Mollusk would be directly descended from the Turbellarian. This 

 view may be contrasted with that which regards the Trochophore (includ- 

 ing the Veliger under this term for convenience) as the ancestor by means 

 of the following scheme: 



TUEBELLAEIAN THEORY. 



Turbellarian larva — .—^ Turbellarian 



(ancestor^i 



""Trochophore 



Annelid 



Mollusk 



TROCHOPHOEE THEORY. 



y^^urbellarian 



Turbellarian larva<<^ 

 (ancestor) xv 



N^Trochophore 



Annelid 



Mollusk 



SUBKINGDOM METAZOA. 



TIPE M0LLU8GA. 



I. Class Amphineura.— Visceral hump not developed; bilaterally sym- 

 metrical; shell represented by scattered spicules or by a series 

 of calcareous plates ; anus terminal. 



