24 ILLINOIS BIOLOGICAL MONOGRAPHS [24 



the lateral edges are frequently encountered but are not as numerous as 

 are the fibers of the muscle sheath; they do not occur in groups but 

 usually singly. In the region of the septa the transverse and dorso- 

 ventral fibers are more numerous and form a sort of muscular plate. 



The muscles of the head are not arranged in the same order as in 

 the Cyclophyllidea, as La Rue (1909) pointed out. In transverse sections 

 of the head the muscles are usually encountered as follows: Near the 

 tip is a rhomboid of muscles, deeper down a transverse muscle cross and 

 then a diagonal muscle cross, the last two forming a muscle star. The 

 muscle star occurs in the region of the suckers but in certain cases 

 (Riggenbach 1896) it may extend below the level of the suckers. Usually 

 as soon as the suckers have been passed the muscles take on an arrange- 

 ment typical of the muscle arrangement in the neck and the young pro- 

 glottids. In longitudinal sections certain heavy muscle fibers are seen 

 to pass from the neck to the surface of the head near the tip, others 

 (the greater number) pass in bundles to the lower and inner surfaces of 

 the suckers. A vertical muscle cross connecting the two adjacent suckers 

 by the lower margin of each sucker to the upper margin of the other is 

 also distinguishable. The muscles of the head have not been studied in 

 many of the species. The writer has studied them in Ophiotaenia fila- 

 roides, Proteocephalus amhloplitis, and P. singularis and to a certain 

 extent in P. pinguis. The muscles of the head of P. singularis are de- 

 scribed in another part of this work (vide infra) mth drawings illus- 

 trating the structures found. /Riggenbach (1896) has studied the muscu- 

 lature of the head of P. fossatus and Coralloiothrium lobosum. In the 

 latter he noted certain variations from the type found in other Proteo- 

 cephalids. These variations* are associated with the type of head found 

 in that genus. 



The genital pore is situated on the lateral margin of the proglottid, 

 right or left. Its position alternates irregularly from one side to the 

 other. For each species the genital pore has a fairly constant location 

 on the margin. In some species it is near the middle, and in other cases 

 it is posterior to the middle, in others anterior. A genital papilla or 

 eminence on which the pore is situated is not usually present. In one 

 species, Ophiotaenia grandis, there is frequently a marked pitting or 

 contraction about the genital pore which causes the latter to be deeply 

 set back from the straight line of the margin. The genital pore leads 

 into the genital atrium into which typically both cirrus and vagina open 

 and to which the outer end of the cirrus-pouch is attached. The atrium 

 is to be considered as an invagination of the outer body wall of the 

 proglottid. 



