CHAPTER VI. 
BRANCH VI.—MOLLUSCA. 
General Characters of Mollusks.—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 Linneus, 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, 7. ¢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 
