GEROULD : CAUDINA. 181 
cipal reasons which lead to this conclusion have been presented in a 
masterly way by Ludwig (91%, p. 492-495 and Bronn’s Thierreich, 
Bd. 2, Abth. 3, p. 448-451). It is therefore necessary in this con- 
nection to bring up only a few points in regard to Caudina, hitherto 
somewhat unsettled, which may bear upon the relationships of the 
Molpadiidae. 
In the first place I have found that there are invariably fifteen 
tentacles, the normal number for the Molpadiidae. This number, as 
Ludwig has suggested, has never been found in the Holothuriidae, 
whereas it occurs in both the Cucumariidae and the Synaptidae. 
The similarity in the arrangement of the muscle layers in the wall of 
the alimentary canal of Caudina and of Cucumaria, in distinction 
from the conditions in Holothuria tubulosa, has already been noted 
(p. 84, 85), and the same has been shown by Danielssen and Koren 
for another representative of the Molpadiidae, 'Trochostoma Thom- 
sonii, Finally the arrangement of tentacles in the interradii in 
Caudina resembles that found in the Cucumariidae rather than the 
conditions which obtain in the Holothuriidae. 
So marked are the differences which distinguish the Cucumariidae 
and Molpadiidae from the Holothuriidae, that Ludwig concludes 
that they together represent one of the two diverging branches of 
the Holothurioidea. The Synaptidae, he believes, are derived 
from the former branch before its divarication to form the Molpadi- 
idae and Cucumariidae. The Synaptidae are distinguished from all 
other holothurians (1) by having uninterrupted circular muscles in 
the body-wall, (2) by the structure of the calcareous ring, (3) by the 
absence of radial canals, (4) by the presence, in some cases, of an 
external longitudinal muscle layer throughout the wall of the alimen- 
tary canal, (5) by the absence of respiratory trees and the presence 
of ciliated urns, and, finally, as I am now able to add with a reason- 
able degree of certainty, (6) by the fact, that in this family are 
found the only hermaphrodites which occur among holothurians. 
Inasmuch as the Synaptidae differ in so many points from other 
holothurians, the idea advanced by Cuénot and others, that they 
have been derived from a primitive form distinct from the ancestors 
of the remaining families, seems not wholly improbable. I am, 
however, inclined to adopt the view of Ludwig, that they represent 
an early off-shoot from the common branch of the Cucumariidae and 
Molpadiidae near its junction with the main stem from which all holo- 
thurians have arisen. The many points of similarity both in gross 
