MUSEUM OF COMPARATIVE ZOOLOGY. 171 



cells recall those of the brain. One finds (Plate V. Fig. 70) the same 

 distribution of chromatic substance, but with more numerous nucleoli, 

 and the same sort of enclosed bodies. One edge of the nucleus shown 

 in the figure is very indistinct, because it was cut obliquely. It recalls 

 the appearance presented by the nucleus of the dorsal cells. The same 

 is true of the nuclei of the large ganglion cells in the ventral nerve cord 

 seen in longitudinal section. 



The female does not show any such extreme modification of the pos- 

 terior end of the ventral nerve cord as was found in the anal ganglion of 

 the male. The only specimen favorable for the study of these relations 

 shows (Plate IV. Fig. 57) a slight swelling in the ventral nerve cord 

 just at its end, which lies below the terminal bulb. There appear to be 

 a very few large ganglion cells at this point, and yet it is an unimportant 

 modification as compared with that of the male. The peripheral layer 

 of ganglion cells, so characteristic of the anal ganglion in the male, 

 seems to be entirely lacking in the female. 



6. Sexual Organs. 

 a. Male. 



In all males one finds a sac suspended from the dorsal line and fill- 

 ing a larger or smaller portion of the body cavity. It shows the char- 

 acter of its walls best when almost empty. Then one sees a fine outer 

 boundary (Plate IV. Fig. 54), with occasional elongated deeply stained 

 nuclei. From this fibres radiate through the cavity of the sac to 

 form a delicate large-meshed network, or the sac may in places be en- 

 tirely empty. These details are all hidden when the organ is filled ; 

 even the walls cannot be demonstrated, although their presence may be 

 inferred from the regular outline of the mass. In this case the sac is 

 enlarged so as to fill a considerable portion of the body cavity. This is 

 true of the anterior or middle region of the body ; farther posteriad 

 the sack seems to become crescentic in cross section, the two horns of 

 the crescent being fastened to the lateral body walls. When the sac 

 here is filled, it occupies the entire space dorsal to the anal ganglion 

 (Plate VII. Fig. 90). 



In most cases the sac is filled with minute oval bodies of uniform 

 size, only 1 jw in diameter. No other structures are constantly present, 

 so that their abundance, minuteness, and uniformity in size and appear- 

 ance render it practically certain that they are spermatozoa, and that the 

 sac is the male generative organ. 



