Nervous System. The nervous system is relatively strongly 

 developed and in some respects it is simplified in comparison with the 

 turbellaria and in other respects it is significantly more complicated. The p. 51 

 cephalic brain among more primitive forms is located dorsally in front of 

 the pharynx (Dactylogyridae, Tetraonchidae and others), and among more 

 highly developed types, above the pharynx itself (Polystomatidae) or directly 

 posterior to it (Microcotylidae and others). For the nnost part, the brain 

 consists of two powerful ganglia joined by a single dorsal commissure 

 (Figs. 73, 74). Among certain forms, head ganglia are joined by means of 

 the dorsal and ventral commissures thus forming a nerve ring around the 

 esophagus (Capsala, Nitzschia and others). Three to four pairs of anterior 

 nerves emerge from the head ganglia, sometimes immediately separating 

 into individual fibrillae and innervating the anterior end of the worm. As 

 a rule, three pairs of nerve trunks ennerge behind the head ganglia: dorsal, 



Fig. 71. Polystoma integerrimum 

 Froelich, head end of an adult worm 

 showing the excretory system 

 (junction of the right and left halves 

 of the ducts in the anterior part of 

 the body). (According to Zeller, 

 1872, simplified). 



Fig. 72. Monocotyle ijimae Goto, 

 structure of the excietory system 

 (the junction of the right and left 

 halves of the ducts ! in the posterior 

 part of the body). (According to Goto, 

 1894). 



lateral, and ventral. As a rule, the ventral pair is the most powerful and 

 often forms a special ring of attaching disc on which are sometimes located 

 the gangliose widenings corresponding to the attaching organs. Among the 

 forms with asymmetrical attaching apparatuses, the ventral nerve trunks 

 are also usually asymmetrical (Fig. 75). Among a number of forms the 

 dorsal pair of nerve trunks is weakly developed and among some of them is 



46 



