UNSEGMENTED WORMS. 187 



domestic animals belong to some of these branches; and 

 because of the tremendous power of reproduction and 

 the striking metamorphoses they have in order to become 

 adapted to their parasitic life. All have non-sexual 

 methods of reproduction. 



PHYLUM IV. Platykelminthes (or Flat-worms). 



205. The flat-worms include three main types which differ very 

 much in appearance and in their manner of living. They get their 

 name from the ribbon shape. 



The simplest division (Planarians) are aquatic free-swimming 

 forms. They are found in stagnant water on the under surfaces 



FIG. 52- Diagrammatic sagittal section of Microstomum, showing a chain of four 

 zooids (individuals) formed by fission, b, brain (the exponents indicate organs of 

 progressively younger individuals) ; d, divisions (dissepiments) ; e, eyespot ; ent, ento- 

 derm; g, gut; gl, glandular cells about the mouth; m", mouth. 



Questions on the Figure. What convinces you that the zooids 

 are of different age? Is the mouth formed from ectoderm or ento- 

 derm? Is the gut a blind sac? Describe the changes that would 

 have to take place when these zooids separate and make inde- 

 pendent worms. Why is this described as non-sexual reproduction ? 

 How does it differ from the non-sexual reproduction in the coelenter- 

 ates? What does sagittal mean? 



of objects. They are covered with cilia, like the paramecium, and 

 glide along through the water or swim vigorously by using the whole 

 body. A general idea of their organs can be had from Figs. 52 

 and 53. 



206. The second group (the Trematodes) are small and usually 

 parasitic animals. Their metamorphosis is very complex and often 

 takes them through two hosts at different periods of the cycle. 



