PAPERS RELATING TO NATURAL HISTORY. 189 



essentially closed, we have no function manifested at that end, except the de- 

 gradational one of adhesion. In the Tunicata (Series I, T) we have, through 

 continued cephalization, the mouth thrown to the bottom of the sac, or nearer 

 the anterior end, and now the anus terminates behind the mouth, and poste- 

 riorly. 



" The heart has also followed the intestine in its rotation and becomes ante- 

 rior, and partially dorsal. The nerve mass is still posterior, and occupies a po- 

 sition between the two openings as in Polyzoa. 



" In these three classes ; namely, Polyzoa, Brachiopoda, and Tunicata, the 

 sac is essentially closed at the anterior end, and consequently the mouth opens 

 toward the posterior end, and with few exceptions all are attached by the ante- 

 rior end. 



"This makes a natural division, corresponding to the Molluscoidea of Milne- 

 Edwards, the Anthoid Mollusks of Dana, and a portion of the neural division 

 of Huxley. In the Lamellibranchiata {Series I, L) we have the sac opening 

 anteriorly, and the mouth permanently occupying the anterior region, though 

 in the lower forms pointing posteriorly, and in all cases the tentacular lobes 

 pointing in that direction, and the mouth bent downward (ventrally), and par- 

 tially obstructed by the anterior adductor, or by the undivided mantle. 



" In Gasteropoda (Series I, G) the posterior end of the sac becomes essenti" 

 ally closed, and the ambient fluid now finds access to the gills through the an- 

 terior (though partially ventral) portion of the sac, while with Cephalopoda 

 (Series I, C) the opening is all antei-ior. 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 opening 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 structure, with the oral and anal openings, 

 lies at the posterior pole of the 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 Lamellibranchiata, the posterior end of the 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 identical, being of an in- 

 fusorial character, and as such it is obvious 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 requires separation and trituration, 



" In nearly all the foregoing homologies, and also the position in which I 

 place the Tunicate sac, I am sustained by the writings of eminent naturalists. 

 "With the Brachiopoda, however, my views completely reverse the accepted poles 

 of the body, though, even here, according to " Woodward's Treatise on Mol- 

 lusca," page 204, Forskahl and Lamarck " compared Hyalea with Terebratula ; 



