MOLLUSCA THE MONOMEROUS TYPE. 195 



CHAPTER XL 



MOLLUSCA. 



THERE is not one of the five great groups of the animal 

 kingdom in which, as in Mollusca, the relation of bilaterality 

 to its subordinate, the type of the grand division, is so clearly 

 lighted up, and yet at the same time in which the former seems 

 to be so lost sight of and obliterated by the excessive development 

 of the latter. We might well say that the Mollusca are char- 

 acterized by the excessive development of uniformity. Among 

 Zoophytes and Articulata we find an excess of repetitions, of 

 multiplicity of parts; but here every part or organ is single, or a 

 single pair, and, instead of repeating itself, it develops its uni- 

 formity to relatively extreme proportions. The Zoo'phytes are, 

 from back to front, dorso-ventrally, polymerous ; the Articulata 

 are, from tail to head, uro-cephally, polymerous ; and the Mol- 

 lusca are monomerous. 



An attentive inspection of these two, (figs. 118, 119,) profile 

 and foreshortened, views of an animal of the lowest group among 

 Mollusca will instruct you as to how plainly bilateral even the 

 least developed of this grand division are ; and now transporting 

 your eyes to another pair of figures, (figs. 124, 125,) which rep- 

 resent a profile and foreshortened view of one of the highest 

 and most completely organized of Mollusca, you will find the 

 same idea none the less manifest. Among the intermediate 

 grades of these two extremes the apparent obliteration of this 

 character is most noticeable ; and in order to unravel before your 

 eyes the obscurity which reigns there, it is necessary to make 

 you acquainted with the relations and nature of the organs of 

 the more simple members of this great group. I shall return, 

 therefore, to the lowest ranks, and make a brief sketch of the 

 Bryozoan organization. 



