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 tentacida, 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. 

 HI. 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 inijjrobable 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 coh)ur 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 



