188 PAPERS RELATING TO NATURAL HISTORY. 



presses the plan, but is equivalent to the titles Vertebrata, Articulata, and Ra- 

 diata, and is in no way a qualitative appellation." 



It appears to us, however, that Prof. Dana's account of the Mol- 

 luscan plan fully justifies the established name, which there would be 

 great inconvenience, and no compensating advantage, in changing for 

 that proposed by Mr. Hyatt. 



Proceeding, now, to the main object of the essay, we endeavour to 

 select what is most necessary for conveying a just idea of the author's 

 views, in a series of short quotations : — 



" In the following considerations, all preconceived ideas regarding the rela- 

 tive positions of the dorso-ventral, and antero-posterior diameters of the ani- 

 mal must be laid aside, and the essential structure of the animal if rightly un- 

 derstood, must be our guide. The gradual morphological changes of the con- 

 tents of the sac, and all other relations, are based on the principle of Oephali- 

 zation. In the plate presented (Series I) I have given a typical figure of the 

 six prominent groups of the Saccata ; * namely, Polyzoa, Brachiopoda, Tunicata, 

 Lamellibranchiata, Gasteropoda, and Cephalopoda. 



" For obvious reasons, only the intestine, head, and pedal ganglia within the 

 sac are represented. These six figures are placed in their normal position, ante- 

 rior pole downward, the dorsal region is turned to the left. Commencing with 

 the Polyzoa (Series I, P) we have the sac closed, while the mouth and anus 

 terminate close together at the posterior pole of the sac ; the mouth occupying 

 the extreme posterior position, and by a dorsal bend of the intestine upon itself, 

 terminate dorsally. The nerve mass is found between the oral and anal open- 

 ings. 



" In the Brachiopoda (Series I, B) we have a permanent invagination of the 

 sac, and the mouth, as in Terebratula, already occupies a position some distance 

 from the posterior edges of the overlapping shells, and the brachial coils perma- 

 nently occupy the space thus made.f 



" We have in this group a dorsal flexure of the iatestine, and a tendency to 

 terminate as in the Polyzoa. In Lingula it terminates posteriorly and at one 

 side. By the permanent inversion of the sac, the mouth makes a great advance 

 toward the anterior pole. 



" (The manner in which I view the Brachiopoda, if true, will entirely reverse 

 the accepted poles of their structure . What has been considered as dorsal, ia 

 here regarded as ventral, and what has been considered as anterior, is here re- 

 garded as posterior. Further remarks on this will be made hereafter). 



" Thus far the balance of structure has been thrown to the posterior pole of 

 the sac, and though we see a cephalization, or concentration of the muscular 

 system and viscera, toward the anterior pole in Brachiopoda, yet that pole being 



» These fi^'ures ai e diagrammatic outlines, enabling the reader more readily to follow the 

 author's ideas. 



t " Terebratulina caput-serpentis, and Crania anomala, projected their cirri beyond the 

 margin of tlie open valves, and moved them as the Polyzoa move their oral tentacles, but in 

 • no instance were the arms extended." Woodward's Treatise, p. 166. 



