168 MORSE, CLASSIFICATION OF MOLLUSCA 



nidae, 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 Unionidae 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 (Series 

 I, G) 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 (Series I, C) the opening is all anteri- 

 or. Thus far we have traced the gradual cephalization 

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

 dotted lines X X, running through the oral open- 

 ing of each figure in Series I of Plate, show the 

 gradual advance of this opening from the lower to the 

 higher classes. In the lowest class all the display of struc- 

 ture, with the oral and anal openings, lies at the posterior 

 pole of sac. In this highest class, all this display of struc- 

 ture 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 ante- 

 riorly as in the Lamellibranchiata, 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 class- 

 es. The nature of their food is also identical, being of an 

 infusorial character, and as such it is obvious that masti- 

 cating 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 be- 

 comes 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 requires separation and trituration. 



In nearly all the foregoing homologies, and also the po- 

 sition in which I place the Tunicate sac, I am sustained 

 by the writings of eminent naturalists. With the Brach- 

 iopoda, however, my views completely reverse the accepted 



