36 
STRUCTURE AND PHYSIOLOGY 
No trace of a nervous or vascular system of any kind has been 
detected, nor is there any organ of sense, but the polypes are 
notwithstanding very sensible of external impressions. # When 
left undisturbed in a glass of fresh sea water, they push their 
tentacula beyond the mouth of the cell by straightening the bo- 
dy ; and then expanding them in the form of a funnel or bell, 
they will often remain quiet and apparently immoveable for a 
long time, presenting a very pretty and most interesting object 
to an observer of “ the minims of nature.” If, however, the 
water is agitated they withdraw on the instant, probably by the 
aid of the posterior ligament or muscle ; — the hinder part of the 
body is pushed aside up the cell, the whole is sunk deeper, and 
by this means the tentacula, gathered into a close column, are 
brought within the cell, the aperture of which is shut by the same 
series of actions. The polypes of the same polypidom often pro- 
trude their thousand heads at the same time, or in quick but ir- 
regular succession, and retire simultaneously or nearly so, but 
at other times I have often witnessed a few only to venture on 
the display of their glories, the rest remaining concealed ; and 
if, when many are expanded, one is singled out and touched with 
a sharp instrument, it alone feels the injury and retires, with- 
out any others being conscious of the danger, or of the hurt in- 
flicted on their mate. 
Of the anatomy of the hydraform polypes a sketch has al- 
ready been given in the beginning of this chapter. They differ 
from the ascidian in their figure, which is somewhat globular or 
cylindrical and straight ; in the position of the body, which is 
vertical ; in the homogeneity of their composition, which is a 
semitransparent glairy gelatine, full of microscopic coloured 
granules; *f“ and very remarkably, in being contractile at every 
que je regarde comme analogues au foie.” — Fig. 2 represents the polype of 
Vesicularia imbricata highly magnified. It is copied from Thompson’s Zool. 
111. Memoir v. pi. i. fig. 4. 
* “ But as we perceive, in these animals, phenomena which take place by 
the medium of nerves in animals of a more elevated order, that is to say, sensi- 
bility and voluntary motion, it is not improbable that in them the nervous sub- 
stance is mixed with their gelatinous or mucous mass, without being demonstra- 
ble as a particular tissue.” — Tiedemann’s Comp. Phys. p. 64. 
t Trembley having ascertained that the colour of the polype resides in these 
granules, and that it varies with the quality of their food, of which the nutritive 
part or chyme passes first into the granules of the stomachal cavity and then 
