9 MANUAL OF THE MOLLUSCA. 
such as Conularia, are rich in the phosphate of lime. The 
digestive cavity is completely separated from the walls of the 
body. The neryous system consists of three pairs of ganglia, 
except in the Brachiopoda, and these neryous centres are very 
much scattered. Hence Professor Owen has proposed the term 
Heterogangliata for the great group of Mollusca. The end of 
the alimentary canal nearest the mouth is surrounded by the 
gangha which supply the foot and head. 
3. The various tribes of insects, spiders, crabs, starfishes, 
echinoderms, entozoa, and worms, have no internal skeleton ; 
but to compensate for it, their outer integument is sufficiently 
hard to serye at once as a support, a covering, and a defence 
for the soft parts. This external armature, like the bodies and 
limbs which it covers, is divided into segments or joints, which 
well distinguishes the members of this group from the others. 
The propriety of arranging worms with insects will be seen, if 
it be remembered that even the butterfly and bee commence 
~ existence in a very worm-like form. This division of jointed 
animals bears the name of the Annulosa. The neryous system 
consists of ganglia arranged in pairs in the middle line of the 
body. From this equal lateral development cf the neryous 
centres Professor Owen calls the groun Homogungliata. The 
neryous system is traversed by the alimentary canal. The 
radiated animals form a part of this sub-kingdom. 
4, The next sub-kingdom comprises most of the polypes, 
such as sea-anemones, the fresh-water hydra, and corals, in 
which the general cavity of the body communicates freely with 
that of the digestive apparatus, on which account they are 
called Coelenterata. The soft parts forming the body wall are 
composed of two distinct membranes; there is no heart; no 
apparent special respiratory organ; and in most cases very 
shght traces of a neryous system. 
5. All the animals not combined in the above groups, such as 
the sponges, the foraminifera, and a large proportion of the 
microscopic animalcules, form the last sub-kingdom, named 
Protozoa. They are characterised by a general absence of any 
special organ. 
There seems to be a much closer relationship between the 
molluscan and the protozoic sub-kingdoms than between the 
molluscan and any of the others. It is always easier to pass 
from the highest part of a sub-kingdom downwards in the scale 
of nature than to pass upwards. Thus we can step from one 
form to another without mecting with any marked distinction 
from the Cephalopods to the Brachiopods, and from them to the 
