170 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
in a considerable number of the Elpidiidae, it is always the left; it 
would appear that the cause which has produced such a condition in 
these forms is the same as that acting to produce the imequality in 
Caudina, Ludwig (81) has found, however, that in Chirodota 
rotifera the right group of tubules is the more fully developed, and 
the same appears to be true of Ankyroderma Jeffreysu, as figured by 
Danielssen and Koren (82, Tab. 10, fig. 15); so that, whatever the 
cause of this inequality in right and left sides, the conditions in 
different holothurians, even though they are closely related forms, 
are unlike. When fully developed the genital tubules fill the larger 
part of the body cavity, extending backward to the base of the respir- 
atory trees. A single tubule with its branches has been figured by 
Semper (768, Taf. 10, fig. 12), and by Kingsley (’81, Plate 2, fig. 13). 
The genital duct (dt. gen.), which is about 2 em, long in a full- 
grown individual, runs forward between the two layers of the 
mesentery (Plate 4, tig. 46; Plate 5, fig. 66) and opens to the ex- 
terior through a single orifice in the conical papilla of the integument 
(pa. gen.), which has been described in connection with the external 
features of the animal. In the sexually mature male (Plate 4, 
fig. 46) it is not of uniform size throughout, its posterior halt 
being distended into a spindle-shaped enlargement 2 mm. in diam- 
eter, while the anterior half is reduced to a diameter of about 
0.5mm. Although there are no external differences in the appear- 
ance of the male and female individuals, the color of the repro- 
ductive organs enables one at a glance to distinguish them; the testes 
are uniformly of a light yellow color, whereas the ovaries are pale 
brown. The color is due entirely to the contents of the tubules, the 
walls of which are somewhat translucent and not pigmented. I have 
arrived at these conclusions after examining scores of living and 
freshly killed individuals and after studying by means of sections the 
sexual organs of a large number of specimens. 
While the sexes have hitherto been shown to be separate in all the 
Molpadiidae except Caudina, there has been some doubt in regard to 
the conditions in this genus. Semper (’68) asserts that Caudina is 
hermaphroditic, and figures a cross section of a sexual tubule showing 
ova and masses of granular matter, nearly filling the lumen of the 
tubules, which he believed to consist of sperm cells. This matter 
was undoubtedly derived from the coagulum that fills the genital 
lacunae. Kingsley (781), on the contrary, makes the statement, 
though without producing any evidence to corroborate it, that in 
