THE GENUS ARACHNACTIS 
161 
concluded that the median marginal tentacle of A. albida does not 
make its appearance until after the fourth couple is recognizable. 
In the youngest example of A. sibogse that I had for study 
(fig. 2) there were four couples of marginal tentacles, each many 
times the diameter of the disk in length, and in addition a single 
small tentacle, little more than a tubercle, situated on one side 
of the median line and dorsal to the member of the fourth couple 
of that side. This last small tentacle is evidently a member of 
the fifth couple of tentacles, its fellow not yet having made its 
appearance, since it is a rule in the Cerianthese that the members 
Fig. 1 Diagram of an oral view of A. albida at the time of the formation of the 
median marginal tentacle. = labial tentacle of second couple; Tm = median 
marginal tentacle; — = first, second, third and fourth couples of marginal 
tentacles (adapted from E. van Beneden). 
Fig. 2 Diagram of an oral view of A. siboga^ at the time of appearance of the 
median marginal tentacle. 
of one side of the body are always a little in advance of those of 
the other side in development. The median marginal tentacle 
was also present and was much smaller than the members of the 
first four couples, but decidedly larger than the representative 
of the fifth couple, its length being about equal to half the dia- 
meter of the disk. It would seem from this that in this species, 
as in A. albida, the median marginal tentacle makes its appear- 
ance only after the formation of the fourth couple and before that 
of the fifth. 
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THE JOURNAL OF EXPERIMENTAL ZOOLOGY VOL. 9, NO. 1. 
