92 



A. E. Yemll — Molluscan Archetype. 



forms among the early larval stages of those groups that 

 undergo a marked metamorphosis, and especially among those 

 larval forms that reappear in groups differing so widely among 

 themselves, when adult, that they must be considered as dis- 

 tinct classes and orders. In the case of the Mollusca, we have, 

 in the form of larva known as the veliger^ and in the slightly 

 younger stages that I have called the proveliger and subvel- 

 iger^"^ organisms that swim free, and often seek their own 

 food, and which, in all respects, seem to have claims to be con- 

 sidered as the nearest living representatives of the ancestral 

 molluscan archetype, or archetypes, for it is quite probable 

 that the different classes of Mollusca have descended from dis- 

 tinctly differentiated, veliger-like organisms. Which of these 

 may have been the most primitive it may not be possible to 

 determine. 



Figure 1. — Schematic mollusk, after Lancaster. A, dorsal; B, horizontal sec- 

 tion ; C, longitudinal section. /, foot ; m, m, mantle ; s, shell ; i, tentacle ; 

 g, gill-cavity; g', gill; a, mouth: e, anus; /i, heart; /i', aorta; A;, nephridi- 

 um; A;', nephridial duct; i, liver; i, gonad; y\ genital pore; n,n\'n!\ gan- 

 glions. 



The archetype suggested by Lancaster was, to a very large 

 extent, based on the actual structure of living chitons, which 

 are peculiarly specialized creeping gastropod mollusks, in 

 which the anus has secondarily acquired a posterior position, 

 and in which the genital and renal organs are symmetrically 

 placed on each side, posteriorly. These are conditions very 

 unusual in adult mollusks generally,t and probably never to be 

 found in the larval forms, unless in those of the chitons and 

 allies (Isopleura). 



In order that the real characters of Lancaster's ^' archemol- 

 lusc " may be clearly understood, I have reproduced here three 



* This Journal, p. 18, July, 1896. 



f Adult bivalves generally have the anus posterior and median, but the nephrid- 

 ial and sexual ducts are ventral and lateral. In the veliger stages the anus is 

 ventral and near the mouth, as usual. It gradually shifts backward as the foot 

 grows larger. 



