The Infusoria are not to be distinguished in their early
stages from Amoebae. Kolpoda-Hke forms, supposed at
one time to belong to the Infusoria, have since been shown
to be the young of Turbellarian Worms. The Infusoria
seem then to be essentially a transition group; so much so
that some naturalists have held that the group is not a distinct division of the animal kingdom, but simply a collection of the young of higher animals. It seems proper to
mention now the necessity of learning the condition of the
young, or embryonic stage of animals, whose origin we are
seeking. Supposing the animal kingdom is really represented by a tree, of which the main branches, twigs, and
leaflets are the orders, families, and species into which
animals are divided, common features of structure in
these groups must not be sought at the ends of the
branches which are far apart, but at the point where the
branches diverge. To make my meaning clear, take the
case of young babies, which look very much alike, but
owing to certain hereditary influences, and the effects of a
different mode of bringing up, can be readily distinguished
later in life. The origin of Worms is not to be sought in
comparing a highly-organized member of the group with
one of the Infusoria, but by placing side by side a simple
worm like the Planaria and one of the Animalcula. The
proofs of the Worms coming from Infusoria are furnished
by the resemblance of the young of the Soft Worms to
existing Infusoria, and the peculiarities of structure common
to both.
WORMS.
By looking at Tree III. we see the root of the Worms divides into two branches,—the Soft Worms (Scolecida) and Sac-worms,—the Soft Worms giving rise to the Articulated Worms, in which are seen the beginnings of the Echinodermata and Articulata, while the Sac-worms are the com-
