MARK: BRANCHIOCERIANTHUS URCEOLUS. 149 
their formation is apparently the same as in Cerianthus. They are sym- 
metrically arranged at the margin of the disk, the hiatus at the posterior 
margin giving the line which connects their several insertions the form 
of an elongated horseshoe. The shortest, and evidently youngest, ten- 
tacles occupy the ends of the two arms of the horseshoe, and are there- 
fore near the posterior margin of the disk; but the tentacles do not 
increase regularly in length toward the anterior margin. There are 
usually one or two pairs of posterior tentacles that are quite short ; 
but the two of a pair are not necessarily of equal length ; the more an- 
terior ones are successively longer and longer until a maximum length 
of abont 125 mm. (in alcoholic material) is reached in about the tenth 
pair from the posterior margin. From this region forward there is a 
gradual and not great diminution in the length of the tentacles up to 
and including the anterior ones. This gives to the whole disk with its 
marginal tentacles, when pinned out, an appearance somewhat resembling 
acommon palm-leaf fan. The marginal tentacles are so crowded that 
they are much flattened at their bases, and overlap one another, as 
shown in the figure on Plate 3; but the shorter tentacles near the 
posterior margin of the disk (one or two pairs) are usually separated 
from the next more anterior ones by an appreciable distance. 
The middle region of the disk is raised into a nearly cylindrical oral 
tube, which in cross section is oval, and bears at and near its summit 
the oral tentacles ; below these there is usually a slight constriction. The 
free end of this oral tube occupies a plane perpendicular to the long axis 
of the column, i.e. it is not oblique, like the disk from which it rises. 
Owing to the obliquity of the disk to this axis, one side of the oral tube 
— the posterior — is much longer than the opposite side. For the same 
reason the posterior face of the oral tube passes almost imperceptibly 
into the disk whereas the lateral and especially the anterior faces 
make with the disk an angle. Viewed from the upper end, the oral tube 
in the fresh specimen presents an oval outline, the oval being nearly 
twice as long in the antero-posterior direction as in the transverse. 
There is no marked difference between the two angles of the mouth; but 
sections may disclose the presence of a siphonoglyph. The oral tentacles 
outnumber the marginal ones, there being about 130 of them. They are 
arranged roughly in quincunx, and occupy four or five rows on the outer 
surface of the upper end of the oral tube. They vary in size, the largest 
being (in alcoholic material) about 30 to 35 mm. in length, and 0.5 mm. 
in diameter at the base. These, too, are so crowded as to be much 
flattened at their origin. 
