CHAPTER IV. 
BRANCH IV.—VERMES (Worus). 
General Characters of Worms.—Having studied the 
one-celled animals, or Protozoans, and the radiated hydroids 
and polyps without a body-cavity, we pass to an assemblage 
of forms which even in the simplest types are seen to have a 
-dorsal and ventral, a right and left side, and a head and tail 
end. It is rare that the form ofa worm is so modified by its 
habits or surroundings but that we are able to call it a worm, 
though when we attempt to draw up a definition of the 
branch or sub-kingdom Vermes, one which shall exclude the 
worm-like Holothurians or the Mollusks, or certain low mites 
and crustacea, or even the Amphioxus, we find it impossible 
to lay down a set of characters which shall accurately and 
concisely define them. This is due to the fact that the worms 
are par excellence a generalized, synthetic type, from which 
the other branches of the animal kingdom above the Protozoa 
and sponges have probably originated. It will be well for 
the student not to trouble himself at first about a definition 
of the branch, but to study with care the leading types, and 
then, in a review of the group, he will have a more or less 
definite idea of the sub-kingdom, and perceive where its bor- 
ders, here and there, merge into other branches, and he will 
be then able to understand the grounds for the speculations 
regarding the phylogeny or ancestry of the other branches, 
which have all an apparent starting-point from low or simple 
forms resembling such worms as we are next to describe. 
As a provisional definition of a typical worm, we may say 
that it is a many-celled, three-germ-layered, bilateral animal, 
with a well-marked dorsal and ventral side and a head and 
tail end, with the body in the higher forms divided at reg- 
