112 GENERAL STRUCTURE OF MOLLUSCA. 
rently imbibed through the whole surface of their bodies 
from the juices in the midst of which they live ; whilst, on 
the other hand, their reproductive apparatus is enormously 
developed, the multiplied segments of the Tape-worm (for 
example) containing this alone, and the head (as it is com- 
monly termed, though really the body) being able to repro- 
duce these to an indefinite extent after they have been 
thrown off. The group of Rorirera, or Wheel-Animalcules, 
which is one of great interest to the Microscopist, also belongs 
to this lower section of the Articulated sub-kingdom. 
106. The general character of the animals composing the 
group or division Moxuusoa, is, in many respects, the very 
opposite of that which prevails in the Articulated animals, 
The body is soft (whence the name of the group is derived), 
neither possessing an internal skeleton, nor any proper ex- 
ternal skeleton. In some of the most.characteristic specimens 
of the group, such as the Slug, there is no hard frame-work 
or skeleton whatever, the body being alike destitute of 
support and protection. In most Mollusks, however, the 
body has the power of forming a shelly covering, which serves — 
for its protection ; but this does not give any assistance in its 
movements by affording fixed points for the attachment of the 
muscles ; in fact, when the animal puts itself in motion, it is 
obliged to make its locomotive organs project beyond the 
shell. We must not regard the shell as an essential part of 
the Molluscous animal ; because there are many tribes entirely 
destitute of it ; and also because sqme of the Articulata have the 
power of forming a shell ($ 102), which bears a close resem- 
blance to that produced by the animals of this group. Not un- 
Fig. 54.—TrsTacetia. 
frequently we see that, of two animals whose general structure 
is almost exactly the same,—as that of the Snail and Slug,—_ 
