44 ON POLYPS. 



a delicate thread, which we are pretty well convinced we have de- 

 tected running round the roots of the tentacles, embedded in a 

 strong circular band of muscle which surrounds the orifice of the 

 stomach, and acts the part of a powerful sphincter in closing the 

 aperture. 



(61.) After the account which has been given of the general 

 structure of the Actinia, the mechanism by which the tentacula are 

 expanded and withdrawn will be easily understood : these do not, 

 like the horns of a snail, become inverted and rolled up within the 

 body, but owe their different states of extension entirely to the 

 forcible injection of water into the cavities which they contain. We 

 have seen already that the interior of each tubular arm communi- 

 cates freely with the space which intervenes between the stomach 

 and the external integument, a space which, at the will of the 

 animal, is filled with sea-water drawn through the orifices seen at the 

 extremity of each arm : when these minute orifices are closed, and 

 the body of the creature contracted, the water, being violently forced 

 into the tentacula, distends and erects them, as when watching for 

 prey ; and, on the other hand, when emptied of the fluid thus 

 injected, they shrink and collapse. This circumstance, so easily 

 seen in the Actiniae, will probably enable us to account for similar 

 phenomena observable in other polyps, the internal economy of 

 which is by no means so conspicuous. 



(62.) The next tribe of polyps which presents itself to our notice, 

 differs widely from the preceding families in outward form, as well 

 as in many important features of internal structure. It would seem, 

 indeed, to comprise animals distinguished from each other by so 

 many important circumstances, and yet so intimately related by ex- 

 ternal configuration, that it is difficult to separate them, or to leave 

 them in the same group. 



It was imagined a few years ago, before accurate researches had 

 been made concerning the internal structure of these zoophytes, 

 that in all the compound species the polyps or mouths of the 

 general mass were in their essential structure analogous to the 

 Hydra, being simple digestive sacs, without more complication of 

 structure than we have found those of the cortical polyps to possess. 

 Recent investigations, however, have shown that amongst the 

 species ranged by Cuvier under the head of Tubular Polyps, 

 " Polypes a Tuyaux" many are exceedingly complex in their 

 organization, possessing the outward form of the simpler kinds, but 



