CHAPTER VI. 

 BEANCH VI. MOLLUSCA. 



General Characters of MoUusks. The characters which, 

 separate this branch from the others, especially the Vermes 

 arc much less trenchant than those peculiar to other groups 

 of the same rank, and indeed the author only retains the 

 Mollusks as a special branch in deference to the general 

 usage of zoologists, believing that the Mollusca are probably 

 only a highly specialized group of Vermes, where they were 

 originally placed by Linnaeus, and bearing much the same 

 relation to the true worms as do the Rotatoria, the Poly- 

 zoa, the Brachiopoda, etc. It will be seen from the fol- 

 lowing account of the mollusks, that they travel along, appar- 

 ently, the same developmental road as the genuine worms, 

 and then suddenly diverge, and the divergence is not an ad- 

 vance in a parallel direction, but if anything the road turns 

 back, or, to change the simile, the branch of the genea- 

 logical tree bends downwards. It is, and always has been, 

 extremely difficult to define the Mollusca, their original- 

 bilateral symmetry being partially effaced in most of the 

 Gastropoda and in some Lamellibranchs, i. e., in those 

 Gastropods with a spirally-twisted shell like the snail, or in 

 fixed bivalve forms like the oyster, etc. The Mollusca are 

 usually defined as animals with laterally symmetrical, un- 

 jointed bodies protected by a shell, with a foot or creeping 

 disk, and usually with lamellate gills, which are folds of the 

 mantle or body-walls. The special organs characterizing 

 the Mollusks are the foot and, in nearly all except Lamel- 

 libranchs, the odontophore ; but the foot of a snail is simply 

 a modified part of the mantle, and in reality in many forms 

 but a specialized ventral surface, as is that of certain non- 

 segmented worms, like the Planarians and Nemerteans ; while 



