Natural History of British Zoophytes. 235 
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 agitat- 
ed 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 tentacu- 
la, gathered into a close column, are brought within the cell, the aper- 
ture of which is shut by the same series of actions. The polypes o 
the same polypidom often protrude their thousand heads at the same 
time, or in quick but irregular 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 conceal- 
ed ; and if, when many are expanded, one is singled out and touch- 
ed with a sharp instrument, it alone feels the injury and retires, 
without any others being conscious of the danger, or of the hurt in- 
flicted on their mate. 
Of the hydraform polypes a sketch of their anatomy has already 
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 homoge- 
neity of their composition, which is a semitransparent glairy gelatine, 
full of microscopic coloured granules;* and very remarkably, in being 
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 demonstrable 
as a particular tissue." Tiedemann's Comp. Phys. p. 64. 
* 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 
gradually into those placed more towards the surface, infers that they are a kind of 
glands or rather vesicles, which have the power of sucking in and again transpir- 
ing the nutritive fluid Hist, des Polypes, p. 132. Lamarck adopts this opi- 
nion, Anim. s. Vert. ii. 9, which is probably correct, but it ought to be remem- 
bered that it is somewhat hypothetical. Considt in relation to this subject Ro- 
get's Bridgewater Treatise, Vol. ii, p. 77-8, Cams' Comp. Anat. Eng. Trans. 
Vol. i. p. 25, . 23 ; and the reader will find Edwards' and Dutrochet's opinions 
on the nature of the elemeritaty corpscules in Bostock's Elementary System of 
Physiology, Vol. iii, p. 348 et seq. Tiedemann sums up our actual knowledge in 
the following sentence " In animals of a simple structure, polypi, entozoa, and 
some others, in which no vascular system for the movement of the humours has 
hitherto been discovered, the nutritious assimilated liquid passes directly into 
the parenchyma of the body, with which it enters into combination." Comp. 
Physiology, p. 35. 
