No. 2.] STRUCTURE OF TWO FISH TAPEWORMS. 349 



shaped. The size increases very abruptly to .2 mm., then grad- 

 ually narrows down to the genital opening, roughly conforming 

 to the shape of the pouch (PI. XVI, Fig. 17). At the largest 

 part the cirrus in transverse section is .2 mm. in long diameter 

 and .075 mm. in short diameter, being of elliptical shape. The 

 cuticula is .005 mm. thick, and its folds are large and rounded. 

 The circular muscle layer is .002 mm. thick, the longitudinal 

 muscle layer is .0075 mm. thick, and where it dips into the 

 folds increases to .015 mm. The layer of tissue next to the 

 muscles is .02 mm. thick, and around this is the delicate epi- 

 thelium. At the distal end of the cirrus the longitudinal 

 muscle fibers reach nearly to the body cuticula and are inserted 

 in the parenchyma. The greater part of these fibers, however, 

 are reflected back in the longitudinal muscles of the pouch. 

 That part of the cirrus which extends from the distal end of 

 the pouch to the genital pore is a simple hollow stalk with 

 very thick cuticula, and surrounded by the two muscle layers. 

 The cirrus, when protruded, is cylindrical in section and longer 

 than a proglottis (PI. XVI, Fig. 4). In a transverse section of 

 the extended cirrus, proceeding from without inward, the follow- 

 ing layers will be found : a thick cuticula, circular and longitu- 

 dinal muscles, a layer of gland-like cells, then a space, then an 

 epithelial layer, glandular cells, longitudinal muscles, circular 

 muscles, and finally the lining cuticula. By comparing this 

 with a section of the retracted cirrus and with the vas def- 

 erens, it will be seen that the cirrus is evaginated and that 

 the end which was proximal is now distal ; while from the distal 

 end runs back within the cirrus the vas deferens, with its layers 

 in the normal position. By the action of the circular muscles 

 the flat cirrus has become cylindrical, and the pull of the longi- 

 tudinal muscles has turned it wrong side out. That portion of 

 the cirrus which penetrated the body-wall is now seen as a 

 muscular stalk (PI. XVI, Fig. 5). 



A cirrus somewhat resembling this has been described in 

 Diplobothriimi simile by Lonnberg ('92), being simply an invagi- 

 nation of the external cuticula, encircled by its muscle coats 

 and connected with the vas deferens at its proximal end. 

 Riggenbach ('96) describes in P. fossata a cirrus which closely 



