THE GENUS ARACHNACTIS 
165 
in two species of the Siboga collection, as has been in part already 
noted elsewhere, and Cerfontaine (1909) also has recently des- 
cribed it as occurring in C. oligopodus. The discovery of this 
new arrangement of the primary mesenteries is a very important 
contribution to our knowledge of the Ceriantheae, and makes 
necessary a revision of the classification of the group. For the 
arrangement of the mesenteries does not stand alone, but is ap- 
parently associated with another striking structural peculiarity. 
In his report on the Actinian larvae obtained by the Hensen 
Plankton Expedition, Van Beneden (1898) drew attention to the 
occurrence of what he termed acontia in a number of the larvae. 
QO,O.Qo 
QOO.Oo 
Fig. 5 Diagram of the arrangement of the mesenteries of A. sibogae. 
These structures resembled the Hexactinian acontia in being fila- 
ments attached at one extremity to certain mesenteries just below 
the lower end of the mesenterial filament. They are, however, 
very much shorter than the Hexactinian acontia and are limited 
to certain mesenteries. The point which they have of interest 
in the present connection is that in some forms they occur upon 
the mesenteries of the second couple (counting from the mid- 
ventral line), while in others the most ventral acontia occur upon 
the mesenteries of the fourth couple. This latter condition is 
the one which obtains in Arachnatis albida, according to Van 
Beneden's account of that species, and is clearly shown in Boveri's 
Plate XXI, fig. 3. Furthermore, it is the condition that I have 
found in the older examples of A. sibogae (fig. 5). But in these 
