ON THE GENUS LUCERNARIA., ay 
deep, and it may with certainty be affirmed that it is only 
an inversion of the outer membrane, which, indeed, extends 
so far as to penetrate through the gelatinous substance, and 
to form a little projection in the bottom of the cavity of the 
stem, where, though not in the rest of the stem, the inner and 
outer membranes are in contact. Consequently, as, however, 
Lamarck has already justly remarked (a, a, 0), there is, in 
this situation, no orifice communicating with the cavity of 
the body, such as exists, for instance, in Hydra. 
4. Tentacles—In all Lucernarie the tentacles are placed 
in eight tufts round the margin of the bell, which is notched 
between them. In this way the tentacles come to be placed 
on arm-like projections, which in some species attain a con- 
siderable length, and give the bell the appearance of being 
deeply cleft. These arms are not always equidistant, for those 
which arise nearest to the partition between two radial 
canals are placed nearer to one another than those which 
arise in the extent of a radial canal. In this manner the 
arms form four regular groups, and the two arms of each of 
these groups, as may be easily surmised, do not belong to 
one radial canal, but to two contiguous ones; and the two 
arms which arise on the border opposite a radial canal sup- 
port two of such groups. The nearer the two arms in one of 
these groups are approximated, the less deeply is the margin 
of the bell between them notched; the notch, on the other 
hand, being deeper in proportion to the degree of separation 
between the groups. 
Whilst in L. octoradiata and campanulata the arms are 
scarcely perceptibly associated in pairs, and arise at almost 
regular distances apart, it is quite the reverse in L. quadri- 
cornis, in which we have apparently four long arms divided at 
the end. The tentacles (figs. 6, 7), which form a sort of rigid, 
divergent tuft on the extremity of the arm, are, as in all the 
Acalephe, prolongations of the vascular system, and conse- 
quently are constituted of both the outer and inner mem- 
branes. This condition is easily observed in young tentacles, 
in which the gelatinous substance may often be seen between 
the two membranes. But in older ones, on the contrary, the 
in nermembrane is changed, towards the interior, into a dense 
cellular tissue, in which the central canal scarcely remains 
open, whilst on its outer side it is transformed into muscular 
fibres, which run in a longitudinal direction, and form a 
cylindrical layer, from which the tentacle derives its contrac- 
tility. 
The teutacles, of which I counted 25 to 27 in L. octora- 
