36 HARLEY N. GOULD 



posterior angle of the mantle cavity, where the duct ends in a 

 clump of cells touching the surface epithelium. The cells in this 

 strand are easily distinguishable from the germ cells (fig. 62); 

 they are more like the follicle cells of the gonad, and like them 

 have no visible cell boundaries at this time. The cord is sur- 

 rounded by a connective tissue sheath. 



A lumen later appears in the middle of this strand (fig. 63) 

 and the nuclei become definitely arranged around the hollow 

 passage. A duct thus arises out of a solid cord. The walls 

 subsequently become more flattened, as do also the nuclei within 

 them, and the interior surface becomes more definite in outline 

 (fig. 65). In specimens of C. plana up to 2 or 2.5 mm. in length 

 the distal part of the duct cannot be seen actually to open into 

 the mantle cavity; but where it reaches the epithelium of the 

 latter it ends in a thickening of cells, by which the duct is fused 

 with the epithelium, and in which no passage-way can be de- 

 tected. In larger animals the duct does seem to be open to the 

 exterior, but the aperture is very small. 



Further differentiation of the duct depends entirely upon the 

 sexual condition of the animal. In the neuter specimens the 

 gonad is still found in this simple, rudimentary condition unless 

 the individual is large enough for the first signs of female de- 

 velopment to appear. The only change is a certain amount of 

 thickening of the wall and the appearance of a few long cilia on 

 the free inner surface of the cells. These long cilia are charac- 

 teristic of the goniduct at all later stages up to the female phase. 

 They are found throughout its whole length (though at certain 

 periods they are lost in some regions). In fixed material they 

 are not always regularly arranged, and while cilia in general do 

 fix poorly, the irregularity of those in the goniduct cannot be 

 entirely attributed to the action of the preparation fluids, for 

 the intestinal cilia in the same slides are usually very w^ell fixed, 

 and their arrangement is as regular as the teeth of a comb. 

 The length of the cilia in the goniduct is so great that they can- 

 not be extended at full length except in an empty seminal vesicle, 

 but reach to the middle of the passage from all sides, meet 

 there and turn parallel to the walls, all extending in the same di- 



