LUCERN»ARI^ AND THEIR ALLIES. 47 



inrolling of the tentacles, and a subsequent folding together of tlie sides of the um- 

 bella {/i(js. 14, 15); whilst the activity demonstrates itself by a wide expansion of 

 the body, a reversion of the periphery of the umbella, and an extreme elongation 

 of the tentacles (fig. 1). Any one of the latter is movable singly, or a number 

 of them follow each other in one file, with a succession of quick, abrupt flexions of 

 the tip toward the base, or, again, the whole bunch is carried toward the mouth 

 by the arm-like corner of the umbella ; especially if the tentacles have come in con- 

 tact with an object fit for food. The prehensile apparatus in this operation is to 

 be found in the urticating organs {cnvhv, or nernatocysts). Their structure and 

 mode of action are fully described in the chapter on Histology (Cluiptcr VII). 

 Normally a tentacle is single, but now and then one encounters a double specimen ; 

 it may be forked from near the base, or at the ends, only the globose tips being 

 double, sometimes one spheroid supports another on its side {fiy. 38), or at some 

 point of its periphery. Our figure {fid. 54, C) represents a tentacle which is double 

 for about half its length. The prongs of the fork are scarcely smaller than the 

 main shaft, and do not differ from it or the corresponding region of the other ten- 

 tacles in any respect. 



95. The globose tip {figs. 41, 49, 54, (|)^) is always broader than long, and is set, 

 like a cap, on the end of the shaft, in such a way that the axis of the one coincides 

 with that of the other. The form of the spheroid varies to some extent, but always, 

 it would seem, at the end, where, at one time, it projects from the convex surface 

 like a very low cone {fig. 42), or, at another time, it is considerably depressed {fig. 

 41) below the general level. The latter is particularly noticeable in the oldest 

 tentacles, the depression being carried to such a degree that the spheroid has rather 

 the appearance of a very thick disk with rounded edges. The thinness of the walls 

 in such cases, and their consequent semi-transparency, has given rise to the asser- 

 tion, put forth by some naturalists, that the tip of the tentacle is perforate. But 

 that this is a mistake may be proved, not only by careful inspection, especially of 

 the younger specimens, but also by the fact that the depression is frequently at the 

 side {fig. 39), and, moreover, two and even three {fig. 40) occur at one time at 

 diverse points on the periphery. They are formed at will, and are evidently due 

 to the action of the muscle which immediately underlies the outer wall. The 

 opacity of the spheroid of the largest tentacles is so great as to render it com- 

 pletely impervious to the powers of the microscope, unless it be compressed into an 

 unnatural condition. Younger tentacles, however, are more transparent and allow a 

 thorough investigation of this part without subjecting them to any artificial prepa- 

 ration. Its prickly and dotted appearance {figs. 43, 44) is produced by numerous 

 nettling organs {iiematoeysts) which are embedded just beneath the surface, and the 

 color and opacity are owing principally to the intercellular pigment masses which 

 lie at the inner face of the exterior wall. This accounts for the fact that they are 

 darker near the centre than at the periphery. 



96. The walls of (lie tentacles are identical in number with those of the founda- 

 tion from which these cylindrical shafts arise ; but they differ considerably in cer- 

 tain qualities, such as proportionate thickness, and some minor details of organiza- 

 tion. We have, then, in tlieni direct prolongations from the opsophragma {*\ 61), 



