No. 2.] STRUCTURE OF TWO FISH TAPEWORMS. 347 



crowded at all times enough to give each a polygonal shape. 

 They vary in size from .05 to .065 mm. Each is surrounded 

 by a very delicate structureless membrane, which is continuous 

 with the wall of the vas deferens and vasa efferentia. The 

 testes are the first organs to mature and the first to degenerate. 

 The spermatozoa are collected by very minute ducts, which take 

 a general direction towards the center of the proglottis, joining 

 with others in their course. The larger ducts, which finally 

 meet, are about .01 mm. in diameter, and these, after making 

 more or less intricate coils, unite to form the distended vas def- 

 erens, which is .05 mm. in diameter (PI. XVI, Fig. 9, v.d). This 

 is also coiled and twisted, forming a mass a little anterior to the 

 center of the proglottis, extending to the cirrus sac on one side, 

 and from the dorsal to the ventral body-walls. The vas deferens, 

 in addition to the tunica propria, possesses a delicate epithelial 

 layer. As will be seen by comparison, the mass of the coils of 

 the vas deferens in this species is much larger and more com- 

 pact than in any form yet described. At all times in ripe 

 proglottids the vas deferens is filled with mature spermatozoa 

 in thick bunches. Just before it enters the cirrus pouch the 

 duct decreases in size to a diameter of .015 mm., and the wall 

 becomes thicker and acquires a layer of longitudinal muscles. 



The cirrus pouch is roughly wedge-shaped, with the apex at 

 its distal end. Its wall is .0035 mm. thick, composed of a 

 longitudinal and a transverse muscle layer, and lined and cov- 

 ered with a delicate epithelium. It is intimately connected 

 with surrounding tissue and never shifts its position. The 

 wall of the sac extends distally to within .06 mm, of the body 

 margin, then turns inward, and the longitudinal muscle layer 

 is reflected back in the wall of the cirrus. It is in the action 

 of these reflected muscles that the cirrus pouch aids the pro- 

 trusion of the cirrus. When the cirrus is protruded the con- 

 tents of the cirrus sac are fairly pulled away from its walls, 

 which are never moved. 



The vas deferens, where it enters the cirrus pouch, is .01 

 mm, in diameter, and has a wall ,003 mm. thick, with faintly 

 marked muscle fibers. Just within the pouch it is surrounded 

 by a cluster of glandular cells. The duct slightly enlarges to 



