ORDER ACTINOIDEA. 33 



number to the intervals between the fleshy lamella? of the visceral 

 cavity, and often equal them ; and when these lamella?, in the latter 

 case, increase in number, as they do with the growth of the animal, 

 the new interval has soon its new tentacle, with which it communi- 

 cates. 



The tentacles are commonly described as prehensile in their nature. 

 In some species, they are long and flexible, and are well adapted to 

 render aid in capturing the food of the polyp. They sometimes have 

 the power of stinging the hand, and when without this power, they 

 frequently stick closely to the fingers if handled ; and when their 

 prey comes within reach, they close upon it, and force it into the 

 mouth, usually open for its victim. In many species, the tentacles 

 are quite short, or are reduced to mere tubercles ;* and it is, therefore, 

 probable that the passage of the imbibed water, contributing to the 

 aeration of the fluids within, is often their more important function : 

 in some instances they appear to subserve this purpose alone, being 

 well adapted by their texture both to act on the external waters and 

 upon the internal that may find passage through them. 



The tentacle, as above described, is the most common variety. 

 There are others, both simple and branched, that are furnished with 

 suctorial vesicles, like the cup-suckers of the sides, which seem to aid 

 them in clinp-ino; as well as sometimes in their movements. There 

 are still others, which have a minutely divided or lobed surface, and 

 look as if covered with delicate embroidery, as shown on plate 5. 

 This peculiar structure seems intended to enlarge the surface of these 

 organs, and fit them more perfectly to aid in aeration. The circle 

 of them has much resemblance to the branchial rosettes of a Holo- 

 thuria. 



'25. The stomach and visceral cavitij occupy together the whole 

 interior of the animal. The stomach is nearly cylindrical, and 

 extends generally about three-fourths of the way to the base of the 

 expanded animal. It has often a plaited or striated inner surface, and 

 may be closed or opened by a muscular arrangement at bottom, where 

 it communicates with the visceral cavity. Under the microscope, the 

 surface in the A. marginata appeared smooth and covered with vibra- 

 tile cilia; it was also invested with small slightly-projecting points, 

 which become detached, on slight pressure, in the form of vesicles. 



* This is the case in the Actiniae, plate 2, fig. 16, and plate 4, fig. 32 ; and in many 

 species of the genus Fungia, as shown on plates 18 and 19. 



9 



