V PHYLUM PLATYHELMINTHES 265 



been made in the descriptions of the examples — presenting many 

 varieties, filling up the interstices between the organs and leaving 

 only, in some instances, very small spaces — sometimes regarded as 

 representing the body-cavity, or ccelo'me, which we shall meet with 

 in other groups of worms. Sometimes the parenchyma appears to 



!/m- 



FlG. 210. — Parenchyma of Sistomum. a, b. intercellular spaces ; lyin. basement membrane ; 

 c. nuclei ; d. nuclei ; ep. epidermis. (After Braun.) 



consist of distinct large cells with greatly vacuolated protoplasm, 

 with interspaces here and there in which groups of rounded cells 

 are enclosed. Sometimes the constituent cells run together, and 

 the parenchyma then appears as a nucleated, finely fibrillated, 

 vacuolated mass in which the boundaries of the cells are not 

 recognisable. Pigment occurs in the parenchyma in some Khab- 

 doccele Turbellarians and a few Monogenetic Trematodes. In 

 some Turbellaria— species of Convoluta and Vortex — the paren- 

 chyma contains numerous cells enclosing chlorophyll or xantho- 

 phyll corpuscles ; these are symbiotic unicellular Algae, similar 

 in their mode of occurrence to the yellow cells which have been 

 referred to as found in the Radiolaria. Running through the 

 body, for the most part in a dorso-ventral direction, are numerous 

 slender muscular fibres, the fibres of the 'parenchyma muscle; 

 many of these become inserted externally into the basement 

 membrane. 



Great differences exist between the various groups of Platy- 

 helminthes as regards the development of the alimentary 

 system, differences which are, broadly, to be correlated with 

 differences in the mode of nutrition. Some of the Flat-worms 

 — the Turbellaria and some of the Monogenetic Trematodes — 

 procure their food, in the shape of small living animal or vege- 



