26 ILLINOIS BIOLOGICAL MONOGRAPHS [26 



Since the outer end of the cirrus tube is fastened to the mouth of 

 the cirrus-pouch the cirrus is everted when protrusion takes place. The 

 protruded cirrus is then of double thickness, being made up of the heavier 

 walled outer tube and the thin walled inner tube. In certain species of 

 Ophiotaenia, viz., 0. marenzelleri and 0. trimeresuri, one to three or 

 more coils of ductus ejaculatorius are forced out into the swollen base of 

 the cirrus. This could only occur in species in which the ductus has a 

 considerable length. Species having a short straight ductus would not 

 show this character. In the cases above cited the cirrus-pouch is not evag- 

 inated as Schwarz (1908) stated was true of 0. marenzelleri. This con- 

 dition of the cirrus and ductus is more fully discussed under the descrip- 

 tions of 0. marenzelleri and 0. trimeresuri. (See also Figs. 106, 108, 

 199.) Such a pushing out of coils of ductus into the cirrus is not known 

 among the species of Proteocephalus, Choanotaenia, Crepidobothrium, or 

 Monticellia. The writer has recently seen it in some proglottids of a 

 species of Ophiotaenia from a king snake from Florida. 



In certain species, notably P. fallax (Fig. 57) a set of strong circu- 

 lar muscles and a constricted area may be found about the middle of 

 the cirrus-pouch. The inner end of the pouch may also be swollen. The 

 presence of these muscles together with the constricted area which is not 

 usually seen and the swollen inner end suggest a possible explanation 

 of the process of cirrus protrusion. In all species the circular muscles 

 of the sheath or pouch are fairly well developed while the longitudinal 

 muscle fibers have a weaker development. A contraction of the circular 

 muscles and perhaps also of longitudinal muscles would produce a rela- 

 tively high pressure upon the contents of the cirrus-pouch. The inner 

 end of the pouch is closed, the outer end is opened broadly and at the 

 edges of this broad opening the cirrus walls are attached. It is evident 

 therefore that the only escape for the contents which are under pressure 

 is by means of this open end of the pouch through which the cirrus is 

 everted. As an aid in the beginning of the act of protrusion, relaxation 

 of the muscles of the cirrus and a contraction of the weak muscles which 

 connect the cirrus to the cirrus-pouch may play a part. This action 

 once initiated the hydraulic pressure put upon the contents of the cirrus- 

 pouch by the contraction of the muscles of the pouch would complete 

 the act of eversion. In certain cases where the cirrus and the ductus 

 ejaculatorius form a straight tube through the cirrus-pouch it seems 

 probable that a shortening of the pouch is also necessary. Such a short- 

 ening has not actually been observed. 



The retraction of the cirrus is not so easily explained. It seems that 

 strong contractions of the longitudinal muscle fibers within the walls of 

 the cirrus and ductus ejaculatorius would be of great assistance in 



