260 ZOOLOGY SECT. 



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 ccelome, which we shall meet with in other 

 groups of worms. Sometimes the parenchyma appears to con- 

 sist 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 Rhab- 

 docoele 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- 

 table organisms, or floating organic debris, by their own active 

 efforts. Others the Digenetic Trematodes and the Cestodes 

 having reached a favourable situation in the interior of their host, 

 remain relatively or completely passive. An alimentary canal is 

 completely absent in the last-named group, nutrition being effected 

 by the absorption of digested matter from the interior of the 

 animal in which the Cestode lives. In all the rest of the Platy- 

 helminthes there is an alimentary canal, which never opens on the 

 exterior by an anal aperture. All the Turbellaria (except some 

 Acoela) and Trematoda have an alimentary apparatus consisting 

 of two well-defined parts a muscular pharynx and an intestine. 

 The pharynx is usually a rounded muscular bulb, but is sometimes 

 (some Turbellaria) of a cylindrical shape ; it is usually capable of 

 eversion and retraction. Actinodactylella (Fig. 205) is exceptional 

 in having, in addition to a large muscular pharynx, an extensile 

 proboscis with a pin-shaped style, which becomes retracted within 

 the opening of the mouth. Unicellular glands open into the pharynx 

 in most cases. 



The mouth is always ventral, but varies greatly in its position 

 on the ventral surface, being sometimes central, sometimes situated 



