250 ZOOLOGY 



SECT. 



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 

 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 Rhab- 

 doccele Turbellarians and a few Monogenetic Trematodes. In 

 some Turbellaria species of Convoluta and Vortex the paren- 

 chyma contains numerous cells enclosing chlorophyll corpuscles ; 

 these are symbiotic unicellular Algas, 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 in- 

 serted 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 

 difference's 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 animals 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 and Trematoda 

 have an alimentary apparatus consisting of two well-defined parts 

 a muscular phorynj 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. 

 Actinodactyldla. (Fig. 191) is exceptional in having an extensile 

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

 the opening of the mouth. Unicellular (" salivary ") 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 behind, 

 sometimes in front of, the middle of the length of the body. 



In the most lowly organised group of Turbellaria the intestine 

 is represented merely by a vacuolated nucleated mass of proto- 

 plasm without lumen. In the others it is sometimes a simple 



