RELATIONS AND DIVISIONS OF THE CLASS INSECTA. 7 
pair are cliitinous and cover the posterior pair, which 
are, as a rule, ample and membranous, but are some- 
times absent or rudimentary ; the circulatory, alimen- 
tary, and nervous systems are vei'y distinct. 
The sixth division of the Invertebrata, the Mollusca, 
may be defined as soft-bodied animals, with a ventral 
foot and usually a calcareous univalve, bivalve, or 
tubular shell ; they have no articulated limbs ; a com- 
pact heart is always present, driving the blood through 
the vessels into the organs ; the nervous system in its 
highest development consists of three principal ganglia, 
or groups of ganglia, which are reduced to one in the 
lower forms ; the nutritive organs occupy the greater 
part of the body; in habit the species are both 
terrestrial and aquatic, the land species breathing air, 
and those that live in the water having branchiaa or 
gills ; the latter are by far the most numerous por- 
tion, some inhabiting salt water, others fresh : the 
Mollusca are divided into several groups, but we need 
not go into them further than to say that the best 
known are perhaps the Snail, Whelk, Oyster, Cuttle- 
fish, Nautilus, and the Ship-worm or Teredo ; the 
tiny Pteropods, which form the chief food of the 
whale, also belong to this section. 
We need not here do more than just notice tho 
Molluscoidea, which contain the Polyzoa (Sea-mats, 
&c.) and tho Bracliiopoda ; by the most recent authors 
they appear now to be included under or placed in 
proximity to the Vermes. 
The ToNiCATAare exceedingly interesting as forming 
one of the connecting links between the Vertebrates 
and the Invertebrates : this is, however, chiefly notice- 
able in their early development, which presents a great 
