164 
J. PLAYFAIR McMURRICH 
been pointed out that in this same larva there was a single repre- 
sentative of the fifth marginal couple and that the median margi- 
nal judging from its size, is earlier in its development. It seems 
probable that the fifth marginal and fourth labial couples develop 
simultaneously and therefore the median marginal probably 
precedes in its development the fourth couple of labials. 
There is essential agreement, therefore, in this respect between 
A. sibogse and A. albida and a decided difference from what ob- 
tains in the other two species. Unfortunately nothing is at present 
known as to the relative times of development of the labial couples 
as compared with the marginals in A. brachiolata and A. bournei, 
nor is it known whether any of the labial couples appear before 
the third marginals in A. albida and A. sibogse. The two labial 
couples in Boveri's youngest albida larva may both have appeared 
subsequently to the development of the third marginal couple, 
and if this be the case there may be an identity in the order of 
appearance of the labials as compared with the marginals in all 
four species. But whether this prove to be so or not, there is a 
striking difference in the development of the labials at the time 
when the median marginal develops. 
Turning now to the arrangement of the mesenteries I wish to 
call attention to a peculiarity which occurs in A. albida and A. 
sibogse and has not hitherto received the recognition that it de- 
serves. Unfortunately it is not possible at present to definitely 
assert its absence in the other two species. 
The ventral couple of mesenteries in all Cerianthea3 is associ- 
ated with the single siphonoglyph and the mesenteries composing 
it are in all cases short, sterile and destitute of mesenterial fila- 
ments. The members of the succeeding couple, on the other hand 
in the majority of species at present known, are very long, extend- 
ing to the aboral extremity of the body, and are fertile and provided 
with mesenterial filaments. These mesenteries have been termed 
the continuous mesenteries, an expression which may conveniently 
be replaced by telocnemes. In 1904, however, Roule described a 
form which departed from this arrangement, establishing for it 
the genus Pachycerianthus. In this the telocnemes are the fourth 
couple and not the second, and I have found the same arrangement 
