A. E. Verrill — Molluscan Archetype. 



93 



of his diagrammatic figures. (Fig. 1, A. B. C.) This hypo- 

 thetical form has a convex or low dome-like body with a sim- 

 ple conical dorsal shell, partly enclosed in a dorsal mantle 

 cavity, instead of the series of eight shelly dorsal plates found 

 in chitons. (Fig. 2.) It has a well-developed, broad, ventral, 



Figure 2. — One of the Chitonidse (Trachedermon exaratus, from life). A, doraal 

 and B, ventral views; a, mouth; e, anus; /, foot; gr', gills; y\ genital pore. 



Figure 3. — Neomenia carinata. Ventral view, a, groove or rudimentary foot; 

 6, mouth; c, anal area and rudimentary mantle. 



creeping foot, like that of chitons and many other gastropods. 

 It has a single posterior pair of pectinate gills (tig. 1, g')^ 

 instead of a row on each side, as in chitons. It has the head 

 and tentacles (t) of a normal gastropod, instead of the degen- 

 erate or abortive condition of these organs found in the 

 chitons. (Fig. 2, B.) Even should we admit that this may 

 represent the primitive form of true gastropods, it would seem 

 impossible to derive from it a cephalopod or a bivalve, for the 

 parts are represented as so highly modified and specialized that 

 it would require us to imagine a backward development in 

 order to return to a specialized condition of head, foot, shell, 

 gills, intestine, etc., in order to find a starting point from 

 which the bivalves and cephalopods might have been derived. 

 But the chitons, themselves, undergo a marked metamorpho- 

 sis and have a veHger stage somewhat different from that of 

 ordinary marine gastropods. Therefore, if we are to consider 

 this group as a primitive one, even among gastropods, it would 

 be more natural to believe that the larvse of chitons approach 

 more nearly to real archetypical forms than the adults do. 

 This is doubtless true, for the chiton larva is a peculiar and 

 rather simple veliger, with a rudimentary velum and a simple 

 band of cilia. It has, at first, only a simple shell-gland, like 

 the veligers of other gastropods. Its ventral, or foot-region, 

 early acquires a great longitudinal development, so that when 

 the anus first appears it is situated farther back than usual. 



