CHAPTER VI 



THE ACOELOMATA: PLATYHELMINTHES 



Under this title are grouped the phyla Platyhelminthes, Nemertea, 

 Rotifera, Nematoda, Gastrotricha, Acanthocephala and Nemato- 

 morpha (the three last of which are very small groups). The animals 

 contained in these are unsegmented forms with mesenchyme (p. 129) 

 and the space between the gut and the body wall (when it exists) 

 is a primary body cavity filled with fluid (e.g. Rotifera). The turgor 

 of the body cavity fluid when present has a determining role in the 

 preservation of the form of the body (e.g. Nematoda, and Rotifera). 

 Generally speaking this space with its contained fluid plays the part 

 of a circulatory system, but in the Nemertea the body cavity is re- 

 duced to a series of canals which constitute the first vascular system 

 in the animal kingdom. This primary body cavity has no definite 

 epithelial boundaries and so can be easily distinguished from a true 

 coelom. It tends to be invaded by mesenchyme cells; in the Platy- 

 helminthes these completely fill it, forming a characteristic tissue 

 (parenchyma), and in the Nematoda the cavity appears to be also 

 completely occupied by a very few enormous vacuolated cells: the 

 vacuoles simulate a body cavity. 



The excretory organ is of nephridial type (or it may be derived 

 from this as in Nematoda). It is a canal, closed at the internal end, 

 intracellular or intercellular, with some hydromotor arrangement 

 which maintains a flow of fluid to the exterior. In the simplest cases 

 there is a continuous ciliation of the inner wall of the canal (some 

 Turbellaria). Usually, however, the ciliation has disappeared over 

 most of the canal but is strengthened and diflFerentiated in others; 

 the characteristic units of the system, the flame cells, being now found. 

 Flame cells may be situated in the course of the canal in some forms 

 but usually constitute the terminal organ (Fig. 149). This system 

 though usually spoken of as "excretory" is primarily concerned with 

 the regulation of fluid content and is often absent in marine forms 

 (e.g. Turbellaria Acoela, p. 213). A nerve net is usually present and 

 from this are differentiated an anterior "brain" and some longi- 

 tudinal nerves. The reproductive system is that in which differences 

 between and within the groups principally occur: these differences 

 are to be regarded as adaptations to the varying conditions of life. 



