CHAPTER V. 



FIFTH BRANCH OF ANIMALS. 

 THE WORMS (Vermes). 



General Characteristics. Animals that as a rule have a 

 head, tail, upper and lower surfaces (dorsal and ventral), 

 and generally made up of an indefinite number of joints 

 or segments. An idea of the internal structure can be 

 obtained from the section of a leech (Fig. 48). The di- 

 gestive organ is tubular, extending from the mouth to the 

 vent. The heart is a small pulsating organ above it, and a 

 system of vessels containing a clear red or green fluid lead 

 into the respiratory organs when present ; many worms, 

 however, breathe through the body-wall. The nervous 

 system consists of a white double chain of ganglia extend- 

 ing along the ventral surface. The brain is small, and 

 situated in the upper part of the head. In some, the eyes 

 are in different parts of the body the head, tail, etc. ; in 

 others, they are absent. In the leech, they are confined to 

 the first three segments. In some, as the worms of the 

 first class, two branching tubes are found that constitute a 

 water vascular system similar to that of the Echinoderms. 

 All the worms are developed from eggs, and many pass 

 through remarkable changes before assuming the adult 

 form. The worms include many animals that, though 

 varying greatly in appearance, all possess certain charac- 

 teristics in common. 



