CHAPTER IV. 



BRANCH IV. VERMES (WORMS). 



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 of a 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 



