PLATYHELMINTHES 211 



B. PLATYHELMINTHES, or Flat Worms. 



BY 



J. W. W. STEPHENS, M.D., B.C., D.P.H. 



DEFINITION : Bilaterally symmetrical animals without limbs, the form of which 

 is leaf or tape-like, rarely cylindrical, and whose primary body cavity (segmentation 

 cavity) is absent, the cavity being rilled by a mesenchymatous tissue (parenchyma). 



The mouth is either situated at the anterior end of the body, or is shifted more 

 or less backwards on to the flat ventral surface. The alimentary canal consists of a 



NOTE. An Appendix on Protozoology will be 

 found on pp. 733-752. This has been prepared in 

 order to incorporate a number of new additions to 

 knowledge made since the body of the book was 

 printed off. 



To Binder : face p. 210. 



the capillary processes of which go on uniting into larger branches^nd finally form 

 two large collecting vessels, which, sometimes separately and sometimes united, 

 open to the exterior through one, two, or numerous pores. 



Nearly all the Platyhelminthes are HERMAPHRODITIC, and in .nearly all there are, 

 in addition to the ovaries producing ova, other glands attached to the female genital 

 apparatus, namely, the vitellaria or yolk glands, which provide a substance termed 

 yolk, which serves as nourishment for the embryo. The fully formed eggs have 

 shells and are " compound," i.e., composed of the egg or ovarian cell, which is 

 surrounded by numerous yolk cells or their products of disintegration. The two 

 sexual openings usually lie close together, frequently in the fundus of a genital 

 atrium ; they are rarely separated from one another. Shell glands also usually 

 occur (p. 221). 



Reproduction is sexual, often, however, combined with asexual methods of 

 propagation (segmentation, budding). The Platyhelminthes live partly free in fresh 

 or salt water, exceptionally also on land. The greater part, however, live as 

 parasites on or in animals. 



