31 



CHAPTEE IV. 



WOEMS (vermes). 



Platthelminthes or Flat-Worms. 



Worms are most variable in form, habits, and structure. They 

 are of great interest, owing to their often complex life-histories. 

 To man and his animals they are sometimes deadly enemies, 

 giving rise to such serious and often fatal diseases as Trichinosis 

 and Toeniosis. Nearly every animal harbours one or more 

 vermiceous guests. Some seem to occasion little or no incon- 

 venience to tlieir host, whilst others, if not fatal, are most 

 annoying. There are also worms, such as the earth-worms, that 

 are of the greatest service to man, helping to fertilise the soil. 

 Worms may be found in a great variety of places. Large 

 numbers are marine ; others live in fresh water ; yet others 

 upon land, in damp earth, moss, and in excreta. It is, how- 

 ever, those that lead a parasitic existence, living on some other 

 animal or plant, that we shall have to consider most fully. 



Worms are bilateral animals with unsegmented or segmented 

 bodies. They never possess any jointed lateral appendages, 

 such as we shall see in the group that includes the insects and 

 spiders. A dermal muscular system is developed, and there are 

 present paired excretory tubes or canals. The bodies of worms 

 are elongated, cylindrical, and soft as a rule, adapted to live in 

 damp media. We can always distinguish a dorsal or upper and 

 a ventral or lower surface. Some worms are flat, and are 



