556 
PROFESSOR ALLMAN ON THE STRUCTURE 
presenting a vast multitude of coils rolled together into a complicated mass, as in the 
latter, it has only two or three such coils. Further, when ejected from the capsule 
(while it still holds on by one end to the point of exit) it does not, like the filament of a 
thread-cell, straighten itself and shoot across the field of the microscope, but immediately 
on becoming free coils itself again into a spiral (fig. 10). Indeed I believe that the signi- 
ficance of these pedunculated capsules is something very different from any which has been 
hitherto assigned to the thread-cells ; and it is scarcely possible not to recognize a special 
apparatus of sense in the whole structure just described, including the rod-like tissue in 
which the peduncles of the sacs have their roots, and which is plainly but a modification 
of the structure which forms the claviform or nervous tissue in other parts of the body. 
Indeed it is impossible to overlook the striking resemblance between these pedunculated 
sacs, with their enclosed capsule and cord, and the Pacinian bodies of the Yertebrata. 
If this be a correct view of the nature of the structures here described, we have now for 
the first time evidence which would justify us in assigning a special apparatus of sense 
to a hydroid trophosome. 
But with all this the resemblance between these pedunculated capsules and true 
thread-cells cannot be ignored, and indeed makes us hesitate, even more than we may 
have hitherto done, in regarding the latter merely as urticating organs. It is possible 
that the pedunculated capsules may throw new light on the function and significance of 
thread-cells ; but with no facts beyond those at present before us, we are scarcely in a 
position to speculate further on this subject. 
The best display of the capsules, with their investing sacs and peduncles, was obtained 
from specimens which had been for twenty-four hours immersed in a solution of osmic 
acid of 0T per cent., and afterwards placed in a mixture of 100 parts of glycerine with 
5 parts of acetic acid ; while the most beautiful demonstration of the rod-like tissue was 
found in sections which had been simply macerated in water for twenty-four hours, and 
then examined, without further preparation, under the compressor. The more external 
tissues of the capitulum had been softened and disintegrated by the maceration, and 
were now easily separated by the simple action of the compressor ; while the firm, rod- 
like tissue, offering more resistance to the decomposing action of the water, remained 
beautifully isolated, with its component rods looking almost like the radiating acicular 
crystals of certain forms of zeolite. 
External to the zone of pedunculated capsules is a thin layer of ectoderm, which forms 
the most superficial portion of the capitulum (fig. 3). This is composed of small round 
membraneless cells, containing refringent corpuscles, while the summit of the capitulum 
is almost always occupied by a group of small cells, containing dark brownish-purple 
pigment granules. The two forms of true thread-cells already described are here deve- 
loped in greater numbers than elsewhere, and may be seen scattered, without any defi- 
nite order, among the more superficial cells of the ectoderm. 
