METAZOA MOLLUSCA . 221 



One of the distinctive organs of the Gastropods is the 

 lingual ribbon or odontophore (No. 452, odontophore of 

 Lunatia heros). It is a band armed on the upper side 

 with chitinous teeth and attached at the back of the 

 mouth ; the forward end is free. It is used as a rasping 

 and scraping organ in obtaining food. One of the many 

 classifications of the Gastropods is based on the struc- 

 ture and variations of the odontophore. 



Like Lunatia, Natica hebraea Mart. (No. 453), is a 

 simple spiral but the umbilicus contains a solid pillar or 

 columella round which the whorls of the shell turn. No. 

 454 (Nevcrita duplicata Stimp., see lowest shelf of erect 

 part of Section) shows still better than Lunatia the greatly 

 distended muscular foot. It seems incredible that so large 

 an organ can be contained in so small a shell. 



An instructive series is furnished by Polinices (No. 455, 

 P. mamilla Linn.) : (a) is the young shell showing the 

 open umbilicus ; a wire thrust into this opening reaches 

 upward nearly to the apex; (b) has the umbilicus still 

 visible though the wire does not penetrate so near the 

 apex, showing that the umbilicus is becoming filled by the 

 columella which adheres to it and is spirally twisted ; at 

 this stage the inner lip has a sharp edge. In (c) the 

 umbilicus is entirely concealed by the thickened inner 

 lip ; in this specimen the horny operculum is in place ; 

 the lines of growth near the margin are fine and close 

 together while in (d) which likewise has the umbilicus 

 concealed, the lines of growth are coarser. No. 455 e, 

 is probably a reduced form ; although about a third 

 smaller than (d), its weight is essentially the same, and a 

 thick heavy deposit is laid on over the umbilicus. This 

 series shows that the open umbilicus is a primary con- 

 dition and the concealed umbilicus is secondary. This 

 furnishes a good phylogenetic reason for placing those 

 shells with the open umbilicus as more generalized in a 

 system of classification. 



The next group of plain but tightly coiled shells with- 



