286 Mr. A. Hancock on a species of Hydra 
prehension, according to Corda, who appears to have examined 
these organs with great care, but whose description of one of 
them is erroneous in several respects: the original memoir, how- 
ever, I have not seen. Neither have I had an opportunity of 
consulting Ehrenberg’s account of the minute structure of the 
tentacles of these animals. The smaller vesicles, Pl. VII. fig. 
7 a, and fig. 8, seem to agree with Corda’s description as given 
in Johnston’s ‘ British Zoophytes’; they are elliptical, being com- 
posed of an inner and outer sac, both very delicate, transparent 
and membranous. ‘These bodies are placed with their long axis 
perpendicular to the surface of the nodule in which they are 
imbedded, and have a non-contractile hair-like process projecting 
from the external end. 
The larger vesicles, fig. 7 b, are very complicated, and appear 
to have been only partially understood by Corda, though he had 
arrived at a full comprehension of their functions. They are 
not merely prehensile organs, but are undoubtedly also stinging 
instruments, as supposed by this naturalist, and are at least twice 
as large as those for touch: they, fig. 9, are of an ovate form, 
short and stout, immersed in the substance of the nodule with 
the narrow end uppermost, and immediately beneath the surface. 
At the bottom of the vesicle, which though perfectly transparent 
has rather thick walls, is seen a delicate ling membrane, d, 
folded down upon itself, having a cup- or saucer-like appearance. 
This, according to Corda, is “a saucer-like vesicle.’ Standing 
up from the centre of this is a transparent membranous stalk, 
c, irregularly bulged a little at the sides, and surmounted by a 
sharp arrow-like head, 4, with the barbs much depressed. This 
supporting stalk is described by the naturalist just quoted as 
“a solid, ovate corpuscle.” But to get a full knowledge of this 
apparatus, it must be examined when exserted and ready for 
action. The saucer-like vesicle is then seen to have disappeared, 
and the outer vesicle, fig. 10 a, is lined throughont with a delicate 
membrane, 0 ; in fact the lips or margins of the saucer-like vesicle 
have unrolled themselves and now form the upper portion of the 
lining membrane, the saucer itself being the lower portion of it. 
And moreover the lining membrane is continued through the 
neck of the outer vesicle, and is seen to be prolonged into the 
stalk, c, supporting the arrow-head ; the barbs, ¢’, of which, three 
in number, are now very much elevated, being almost horizontal ; 
and thus protruded beyond the surface of the tentacle are ready 
to lay hold of prey in the manner of a grappling-iron. 
The animal would appear, however, to. have the power of 
throwing the whole apparatus from the tentacle. If a specimen 
be laid on a piece of glass and examined through the microscope, 
a number of these organs with the barbs fully extended will be 
