MUSEUM OF COMPARATIVE ZOOLOGY. 171 
cells recall those of the brain. One finds (Plate V. Fig. 70) the sume 
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 u 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. 
