THE GENUS ARACHNACTIS 
163 
chronously with the members of the third cycle, at least succeeds 
them at a very short interval and undergoes a practically synchro- 
nous development. 
To state the difference in the two groups concisely, the median 
marginal tentacle in A. albida and A. sibogse develops only after 
the fourth couple of tentacles have appeared, while in A. brachiolata 
and A. bournei it develops practically synchronously with the third 
couple. 
The difference in the two groups may also be shown by a com- 
parison of the time of development of the median marginal ten- 
tacle with that of the labials. In all Cerianthese in which their 
development has been observed the labial tentacles lag behind 
the marginals, and this is especially noticeable in the cases of the 
median ventral labial and the first couple, which do not make 
their appearance until several other couples of labials have devel- 
oped. In neither A. brachiolata nor A. bournei is there any trace 
of labial tentacles at the time when the median marginal makes 
its appearance, but in the other two species the case is different. 
In the youngest individual of A. albida studied by Boveri (1890) 
the fourth couple of marginal tentacles was in process of develop- 
ment and there were present two couples of tuberculiform labials, 
corresponding to the second and third couples of marginals. 
In the youngest example studied by Van Beneden (1898), in which 
the median marginal was present, though quite small, there were 
three couples of labials, corresponding to the second, third, and 
fourth marginal couples, those corresponding to the fourth mar- 
ginals being noticeably smaller than the others. Hence it may 
be concluded that in this species the median marginal tentacle 
develops contemporaneously with or just before the fourth^ couple 
of labials. 
In the youngest larva of A. sibogse two couples of labial tentacles 
are present, namely the second and third, and a single member of 
the fourth couple also occurs as a small tubercle. It has already 
^ This enumeration is based upon the order of the interspace to which the 
tentacle belongs, counting; from the mid-ventral line, and not on the order of 
their succession. Thus, the third couple to develop is the fourth couple in the 
final arrangement. 
