ON THE PRINCIPLE OF CEPHALIZATION. 163 



are displayed from the dorsal or ventral valve. Also the 

 Gasteropods, particularly the flat forms like Patella, Chi- 

 ton, etc. and tha Nudibranchs as well, while in the figures 

 of the naked Cephalopods we most usually have a dorsal 

 view. 



Though Prof. Agassiz speaks of radiation as characteriz- 

 ing the Radiates, and similarly of articulation and verte- 

 bration as characterizing the Articulates and Vertebrates, 

 yet Mollusks are spoken of as first introducing the charac- 

 ter of bilaterality, or division of parts along a longitudinal 

 axis, that prevails throughout the Animal Kingdom, with the 

 exception of the Radiates. This then can be no restricted 

 definition for the Mollusca, since it pervades the two high- 

 er branches ; and who will deny the evidence of bilaterality 

 among the Radiates, the higher Echinoderms for instance, 

 as Clypeastroids and Spatangoids, where we have as good 

 a definition of a longitudinal axis, -as we obtain in many 

 Mollusks. Even among the Polyps, as in the Actinaria, 

 the antero-posterior axis is clearly expressed in the undue 

 prominence of the primary radii. 



Prof. Dana has been the first to publicly announce the 

 plan of Mollusca, when he says, "The structure essential- 

 ly a soft, fleshy bag, containing the stomacn and viscera, 

 without a radiate structure, and without articulations.*'* 



As far back as 1855 he has presented this thought in 

 his lectures at Yale College. 



In the year 1832 Mr. AlpKaus Hyatt 'had independently 

 worked out a similar result, and has already in MSS. 

 notes, the necessary data demonstrating the same.f 



Mr. Hyatt also proposes the name Saccata as more fully 

 and truthfully expressing the type, than the unmeaning 

 word Mollusca. This name not only expresses the Plan, 

 but is equivalent to the titles Vertebrata, Articulata, and 

 Radiata, and is in no way a qualitative appellation. 



*Dana's Manual of Geology, p. 148. 



fMr. Hyatt lias relinquished all ideas of publishing on this subject, 

 since becoming aware that I was to do the same. During the prep- 

 aration of these pages, I enjoyed his companionship, and many of 

 the points herein stated, were fully and freely discussed between us, 

 and to him I am indebted not only for the privilege of announcing 

 his proposed name, Saccata, but for the suggestion of certain points 

 to be hereinafter mentioned. 



