OF POLYPES. 



37 



point, so that they can change the figure of their bodies from a 

 globe to a cyhnder, 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. 





^ /tp I 



papillae (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 Elementaiy 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 I'hist. des Polypes, p. 25. Carus' 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. 1-334, pp. 

 371-372. 



f The figures represent Hydra viridis in various attitudes and states. 



