488 
OBSERVATIONS ON THE 
the margin of the aperture of which, their skin has a perfect 
continuity. When in a state of rest or contraction, they 
are doubled in the cell, as exhibited at 5, and its margin 
in this state is obviously inflected. 
The arms^ or tentacula, are nearly cylindrical, and 
limited to the number of eight. 
In some Sertulariae which I have examined, the arms 
seemed furnished on all sides with suckers, analogous 
to those of the cuttle-fish. On the arms of this species, 
however, I detected a very different arrangement, and 
one which I suspect has not hitherto been noticed. 
Each arm is furnished, laterally, with a row of short 
hairs or plates; for the highest magnifier which I could 
conveniently apply did not enlarge the object above a 
hundred diameters, and was incapable of enabling me to 
determine their true shape. The motions of these hairs, 
were, in consequence of the currents which they produced 
in the water, sufiiciently obvious. The hairs, on one side 
of the arm, exercised a continued motion, so as to cause the 
water to flow from its base to the extremity ; while those 
on the opposite side executed a motion the very reverse of 
this, causing the water to descend from the extremity of 
the arm towards its base. And again, if the hairs on the 
right side of one arm were fitted to cause the water to ascend, 
the hairs on the left side of the contiguous arm were 
found suited to produce a current in the opposite direction, 
as exhibited (not from nature, but to render the description 
intelligible) at c, fig. 1. Plate XV. Analogous hairs exist 
on many species of Medusas, Tritoniae, &c. in which thev 
are obviously unconnected with the digestive system as 
assisting prehensible organs, and may probably be con- 
sidered as forming a part of the aerating organs. In this 
Sertularia, their occurrence on the arms, which are true 
prehensible organs, and belonging to the digestive system, 
