BRANCH HERMES: THE WORMS 137' 



slowly and it is several years before they attain their full 

 size. Leeches are long-lived animals, some being said 

 to live for twenty years. 



Flat worms (Platyhelminthes). TECHNICAL NOTE. 

 Collect some live fresh-water planarians (see fig. 30), which are to be 

 found on the muddy bottom of most fresh-water ponds, and examine 

 them while alive in watch-glasses of water. Make drawings show- 

 ing the external appearance, and as much of the internal anatomy 

 as can be seen. The branching alimentary canal can be seen in 

 more or less detail, and with higher power of the microscope parts 

 of the nervous system can be seen also. Have also a tapeworm 

 preserved in alcohol or formalin to show the very flat and many- 

 segmented body. 



The flatworms include a large number of forms which 

 vary much in shape and habits. They are all, however, 



FIG. 30. A fresh water planarian, Planaria sp. (From a living specimen.) 



characteristically flat; in some this condition is very 

 marked. Some are active free-living animals, as the 

 planarians (figs. 30 and 31), while many live as parasites 

 in the alimentary canal of other animals, as do the sheep- 

 fluke and the tapeworms. 



The fresh- water planarians (fig. 30), which live com- 

 monly in the mud of the bottom of ponds, are small, 

 being less than half an inch long. They are very thin 

 and rather broad, tapering from in front backwards. On 

 the upper surface near the front they have a pair of eyes ; 

 the noouth is on the under surface a little behind the 

 middle of the body. The alimentary canal is composed 

 of three main branches, each with numerous small side 

 branches. One main branch runs forward from the 

 mouth, and the other two run backwards, one on each 

 side of the body. There is no anal opening, and the 



