RELATIONS AND DIVISIONS OP THE CLASS INSECTA. 3 
protecting their bodies by a bard shell usually com- 
posed of carbonate of lime ; and the Infusoria, which 
include several well-known microscopical objects such 
as the Vorticella, or Bell Animacule, the Paramoecium, 
or Slipper Animalcule, and others ; the Bacteria also, 
which have obtained such a bad reputation as being at 
the bottom of all putrefaction and probably of all 
infectious diseases, must be classed with the latter 
members of the division. 
The Ccelenterata differ from the preceding section 
in that they have a body cavity which serves alike for 
circulation and digestion ; there is, however, only one 
opening to the body ; no circulatory system, and in 
most cases no traces of a nervous system are present ; 
of the Ccelenterata proper the best known examples 
are the Sea-anemone, the Jelly-fish, and the various 
species of so-called coral “ insects ; ” the Sponges, 
which used to be classed with the Protozoa, are now 
regarded as forming a sub-group of the present division, 
called Ponfera. 
The Vermes are very variable in structure; in fact 
it is hard to find any reliable character to distinguish 
them, except the fact that jointed lateral appendages 
or limbs are always wanting ; as a rule, there is a 
distinct digestive canal, but in the Cestoda it is not 
even indicated, and in the male Rotifers it is rudi- 
mentary ; a vascular system is usually present, but is 
occasionally wanting ; a nervous system is always 
traceable, but is very differently constituted in the 
thirteen classes of which the division is composed ; the 
body is usually elongate, flat or cylindrical, sometimes 
without rings, sometimes ringed, sometimes divided 
into distinct segments; the best known members of 
b 2 
