cavity which changes into a sac close to the pharynx or prepharynx. 



This prepharyngeal sac has the same structure as in Rhabdo- 

 coela. Among a number of species the pharynx can protrude to the outside 

 (the majority of the lower Monogenoidea), but among others it is devoid of 

 this capability. 





•'>>^'.'?; 



Fig. 60. Heteraxine heterocerca (Goto), cross section of the body below 

 the sex aperture. Enlarged 200 times. (According to Goto, 1894). 



Pharynx . The pharynx is usually the very powerful type of 

 pharynx plicatus and pharynx bulbosus of the Turbellaria. Its form is 

 round and somewhat elongated, in rare cases barrel-shaped, egg-shaped, 

 etc. The pharynx is separated from the sac or parenchyma which surrounds 

 it by a special membrane; this same type of membrane limits its interior 

 lumen which often has a tetrahedral outline. The structure of the pharynx 

 is complex and to a known degree resembles the structure of the suckers 

 which can also be explained by a functional similarity (among certain 

 Capsalidae the pharynx even functions in place of the anterior suckers). p. 46 



Usually in the pharynx there is a strongly developed musculature which con- 

 sists for the most part of three layers, external and internal circular layers 

 and the middle radial layer (Fig. 61). Very often there are weakly developed 

 longitudinal fibers. Often the number of the layers increases and the pharynx 

 becomes much more complex. Between the muscular fibers are located the 

 numerous nuclei belonging to the muscle cells and often mononuclear phary- 

 ngeal glands are located in the main part of the pharynx. The latter can lie 

 also in the adjacent parenchynr\a and only pierce the body of the pharynx by 

 their canals. Very often very powerfully developed supplementary glands 

 called salivary or postpharyngeal open into the lower end of the pharynx 

 along with these glands (Fig. 62). Among certain Gyrodactylidae the pharynx 

 is divided into two parts of which the first consists of several pyramidal 

 cells and the posterior part is rounded, usually of muscular structure (Fig. 63). 



40 



