CHAPTEE V. 



PHYLUM PLATYHELMINTHES. 



Vermiform, bilateral, more or less elongated, and usually dorso- 

 ventrally flattened animals, with an anteriorly-placed central nervous 

 mass (cerebral ganglion), and an excretory system of ramified canals 

 containing flame-cells. Enteron ivlien present aproctous. 



The forms included in this phylum are mostly parasitic Entozoa, 

 but some live freely in water or on land. Suckers and hooks for 

 attachment are often present, especially in the parasitic forms. The 

 enteron is absent in one class (Cestoda), and when present is without 

 an anus. Vascular system and body cavity are not found in the 

 group, and the organs are embedded in a plasmatic mass the 

 parenchyma in which muscular and connective tissue elements are 

 differentiated. This is the so-called mesoderm. The excretory 

 system is distributed as a system of branching and often anasto- 

 mosing canaliculi throughout the parenchyma, and in the absence 

 of a vascular system collects the excretory products in all parts of 

 the body. Projecting from the walls of the canals at intervals are 

 thick flame-shaped cilia, and sometimes finer cilia. The canaliculi 

 are supposed to end blindly in hollow cells of the parenchyma, from 

 which cells there often projects into the blind end of the canal a 

 name-shaped cilium. Such terminal cells are called flame-cells. 



In the terms of the cell-theory this system may be described as consisting of 

 a number of branched and hollow anastomosing cells, the spaces or vacuoles 

 within which form a continuous system opening externally, and constituting the 

 cavities of the excretory canaliculi. Such canals are often distinguished as 

 intracellular, and contrasted with intercellular canals, in the walls of which cell 

 limits can be made out. It is extremely doubtful if this distinction has the 

 reality or morphological importance which is often attributed to it. 



The external openings of the excretory canals vary much, both in 

 number and position. 



The animals are usually hermaphrodite, and the reproductive 

 organs are complex. A vitellarium or yolk-gland is very generally 



p 



