OF CONCHOLOGY. 35 1 



gret that want of space prevents us from making more than a 

 very few extracts. 



Mr. Morse, as a preliminary, defines an animal as a sac, the 

 contained mass of structure varied according to the class to 

 which it belongs; thus we have a vertebrate sac, an articulate 

 sac, etc. But, while the terras Vertebrata, Articulata, or 

 Eadiata properly express the plan of structure, the term 

 Mollusca, he objects, does not possess so definative a meaning. 

 He proposes instead, the name Saccata, suggested by Mr, Al- 

 pheus Hyatt, as expressing ih.e plan of structure. " We may 

 remark that the Mollusks as a type, present the sac features 

 most completely, for nowhere do we find the various organs 

 so essentially concealed, or possessing the power of retraction 

 within a sac, as in the Mollusca." < 



"In the following considerations, all preconceived ideas re- 

 garding the relative positions of the dorso- ventral, and antero- 

 posterior diameters of the animal must be laid aside, and the 

 essential structure of the animal, if rightly understood, must 

 be our guide. The gradual morphological changes of the 

 contents of the sac, and all other relations, are based on the 

 principle of Cephalization. . . . 



. . , "Commencing with the Polyzoa, we have the sac 

 closed, while the mouth and anus terminate close together at 

 the posterior pole of the sac ; the mouth occupying the ex- 

 treme posterior position, and by a dorsal bend of the intestine 

 upon itself, terminating dorsally. The nerve mass is found 

 between the oral and anal openings. In this class the mouth 

 and anus have the power of protrusion from the sac. In the 

 three lower orders, Cyclostomata, Ctnostomata, and Cheilos- 

 tomata, the polyzoon, when completely evaginated, presents 

 no fold or inversion of the sac, while in the higher group 

 PhylactoliBmata, there is a partial and permanent inversion of 

 the sac under like conditions. 



"This latter group, combining the permanent inversion of 

 the sac-walls with the lophophoric arms, is the first approach 

 to the Brachiopoda. No organ corresponding to a heart has 

 yet been discovered. In the Brachiopoda we have a perma- 

 nent 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. 



" We have in this group a dorsal flexure of the intestine, 

 and a tendency to terminate as in the Polyzoa. In Lingula it 

 terminates posteriorly and at one side. By the permanent in- 

 version of the sac, the mouth makes a great advance toward 

 the anterior pole. In Terebratula, Waldheimia, and allied 

 genera, where the sac is very short and swollen, and the bra- 



