OF CONCHOLOGT. 359 



tiallj closed at the anterior end, and consequently the mouth 

 opens toward the posterior end, and, with few exceptions, all 

 are attached by the anterior end. 



"This makes a natural division, corresponding to the Mol- 

 luscoidea of Milne-Edwards, the Anthoid Mollusks of Dana, 

 and a portion of the neural division of Huxley. In the Lam- 

 ellibranchiata we have the sac opening anteriorly, and the 

 mouth permanently occupjnng the anterior region, though in 

 the lower forms pointing posteriorly, and in all cases the ten- 

 tacular lobes pointing in that direction, and the mouth bent 

 downward (ventrally), and partially obstructed by the anterior 

 adductor, or by the undivided mantle. The gradual enlarge- 

 ment of the anterior opening is clearly seen, where in the 

 Gastrochsenidse, we have first a minute orifice, for the passage 

 of an immature foot, or metapodium; this opening gradually 

 enlarging in different genera, until in the Union ida3 we have 

 the sac almost completely separated, except dorsally. It will 

 be noticed that the anterior opening is also ventral, or nearly 

 so in the lower forms. In Gasteropoda the posterior end of 

 the sac becomes essentially closed, and the ambient fluid now 

 finds access to the gills through the anterior (though partially 

 ventral) portion of sac, while with Cephalopoda the opening 

 is all anterior. Thus fiir we have traced the gradual cephali- 

 zation of the contents of the sac, and of the sac itself. . . . 

 In the lowest class all the display of structure, with the oral 

 and anal openings, lies at the posterior pole of sac. In this 

 highest class, all this display of structure lies at the anterior 

 pole. Advancing from the Polyzoa, by the gradual advance 

 of the mouth, the posterior pole becomes less prominent. 

 Even when the sac opens anteriorly, as in the Lamellibranchi- 

 ata, the posterior end of sac remains open, and the mouth, 

 partially inclined that way, receives its food from that end; 

 the food being conducted to the mouth by ciliary motion as in 

 the three lower classes. The nature of their food is also iden- 

 tical, being of an infusorial character, and as such it is ob- 

 vious that masticating organs, or biting plates, such as we find 

 in the two higher classes, are not needed. 



"So long, also, as the posterior end of the sac remains open, 

 the anus terminates at that end ; when this opening becomes 

 closed, as in the higher classes, the anus seeks an outlet through 

 the anterior opening, and the mouth, that before received its 

 food from the posterior end of the sac, and by ciliary motion, 

 now distinctly points the opposite way, and is furnished with 

 the proper organs to procure food, the nature of which re- 

 quires separation and trituration." 



Mr. Morse remarks, also, on the various regions of attach- 

 ment in the Classes : — 



