OF POLYPES. 
37 
point, so that they can change the figure of their bodies from a 
globe to a cylinder, or distort it with strictures, and can shorten 
and extend the tentacula at will, sometimes to an extent which 
is astonishing, although nothing like muscular tissue can be de- 
tected in their structure. * When therefore they have occasion 
to conceal themselves within their cells, they are not necessitat- 
ed, like the ascidian, to bend the body in order to obtain suffi- 
cient space for the tentacula, but they shorten the body and the 
tentacula at the same time, causing the one to assume a more 
globular form, and the other to dwindle down to mere knobs or 
Fig. 4. 
papillse (Fig. 4.) f The tentacula, even when fully extended, 
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 tran- 
spiring the nutritive fluid — Hist, des Polypes, p. 132. Lamarck adopts this 
opinion, Anim. s. Vert. ii. 9, which is probably correct, but it ought to be remem- 
bered that it is somewhat hypothetical. Consult in relation to this subject Ro- 
get’s Bridgewater Treatise, Vol. ii. p. 77-8, Carus’s Comp. Anat. Eng. Trans. 
Vol. i. p. 2.5, §. 23 ; and the reader will find Edwards’ and Dutrochet’s opinions 
on the nature of the elementary corpuscles 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. 
* Trembley, Mem. pour l’hist. des Polypes, p. 25. Cams’ Comp. Anat. i. p. 43. 
— Mr Lister, however, says that “ in the substance of the necks of the polypi 
(of Sertularia pumila,) transverse lines were visible, bearing a resemblance to 
those characteristic of voluntary muscles in the higher animals but we may 
doubt whether they are truly muscular, for this accurate observer shortly after 
acknowledges, that “ nothing like muscular contraction was seen in the pulp 
of this (Plumularia setacea,) or any other species.”— Phil. Trans. 1334, pp. 
371-372. 
f The figures represent Hydra viridis in various attitudes and states. 
