we have arrived at a sound
principle, we are therefore unerring in our practice. From ignorance we
may misplace animals, and include them under the wrong division. This is a
mistake, however, which a better insight into their organization
rectifies; and experience constantly proves, that, whenever the structure
of an animal is perfectly understood, there is no hesitation as to the
head under which it belongs. We may consequently test the merits of these
four primary groups on the evidence furnished by investigation. It has
already been seen that these plans may be presented in the most abstract
manner without any reference to special animals. _Radiation_ expresses in
one word the idea on which the lowest of these types is based. In
_Radiates_ we have no prominent bilateral symmetry, as in all other
animals, but an all-sided symmetry, in which there is no right and left,
no anterior and posterior extremity, no above and below. They are
spheroidal bodies; yet, though many of them remind us of a sphere, they
are by no means to be compared to a mathematical sphere, but rather to an
organic sphere, so loaded with life, as it were, as to produce an infinite
variety of radiate symmetry. The whole organization is arranged around a
centre toward which all the parts converge, or, in a reverse sense, from
which all the parts radiate. In _Mollusks_ there is a longitudinal axis
and a bilateral symmetry; but the longitudinal axis in these soft
concentrated bodies is not very prominent; and though the two ends of this
axis are distinct from each other, the difference is not so marked that we
can say at once, for all of them, which is the anterior and which the
posterior extremity. In this type, right and left have the preponderance
over the other diameters of the body. The sides are the prominent
parts,--they are charged with the important organs, loaded with those
peculiarities of the structure that give it character. The Oyster is a
good instance of this, with its double valve, so swollen on one side, so
flat on the other. There is an unconscious recognition of this in the
arrangement of all collections of Mollusks; for, though the collectors do
not put up their specimens with any intention of illustrating this
peculiarity, they instinctively give them the position best calculated to
display their distinctive characteristics, and to accomplish this they
necessarily place them in such a manner as to show the sides. In
_Articulates_ th