THE GENUS ARACHNACTIS 
167 
A. albida and A.sibogsein general form and in the order in which 
their tentacles develop sufficiently demonstrates the necessity 
for separating them from the genus Arachnactis, of which A. 
albida is the type. 
The question as to their proper systematic position must be 
left for future observations to determine, but from what is known 
of their life histories it seems exceedingly probable that they are 
larval forms of species of Cerianthus. Indeed, Van Beneden 
(1898) regards A. bournei as the larva of Cerianthus lloydii, his 
conclusion being based mainly on the fact that the areas of dis- 
tribution of the two forms are essentially the same, and, for a 
similar reason, Kingsley (1904) has suggested that A. brachiolata 
is the larval form of C. borealis Verr. There is much probability 
in these suggestions and the probability will be greatly increased 
if future observations show that the mesenteries of the second 
couple possess acontia in these two species of larvae. 
In conclusion, I desire to thank Professor Max Weber of the 
University of Amsterdam for his courtesy in allowing me to pub- 
hsh in the present paper results that are to a large extent based 
on material contained in the Siboga collections and which will 
be more full}^ considered in a forthcoming report on the Siboga 
Actinians which is now in course of preparation. 
