108 



LUCERNARI/E AND THEIR ALLIES. 



grown, when they may be found measuring in depth several times the breadth 

 ( fills. 88, 89). As to form, the main peculiarity of these cells in an adult tentacle 

 lies in tlieir gradually widening calibre, which is narrowest at the proximal, and 

 bro*;xdest at the distal end. On the whole, then, each cell has the form of a very 

 elongated, prismatic conoid, inverted on its apex; but it is so nwdificd by the lateral 

 pressure of the interstitially imbedded nematocysts (Z, Z') as to be more or less 

 irre"-ular in shape. They diifer also from the cells of the shaft in having a very 

 narrow range of mobility ; in fact they are comparatively fixed, and not subject to 

 wide changes in form as the tentacle expands or contracts. They differ, again, in 

 the position of the nucleus (tZ'), as that is attached much nearer the distal end. 

 They are, besides, more considerably modified in form by the greater amount of 

 granular interstitial matter («"), especially between their proximal basal ends. This 

 is so abundant in very old tentacles as to render their tips nearly opaque ; and, as 

 it varies in color, like the pigment masses in other regions, it gives varied hues to 

 the globose mass. 



203 (A). The interstitkd ci/tohhisiema merits particular attention here, on account 

 of its specialized condition. We have just spoken of the mass of pigment granules 

 in its proximal side, about the bases of the cells ; but here we wish to draw atten- 

 tion to its exclusive devotion, at the distal side, to the formation and development 

 of nematocysts. Not only do these cysts (Z, V) originate in it, and remain imbedded 

 there, sometimes considerably below the general surface of the layer, thus giving 

 to the cytoblastema a specialized character in this region, but they also involve it, 

 indirectly at least, in tlie apparatus of prehension. We refer 'here to the bristling 

 points {fi(j. 88, e^), which are so numerous on the surface, standing singly in the 

 intervals between the cells. From many points of view they project as if in direct 

 prolongation of the axis of a nematocyst ; and frequently, when the shaft of the 

 latter is partially everted, it seems to be the cause of the bristling, appearing to 

 push the plastic blastema before it until it is forced out into a fine point. But 

 that this is not so is shown by the fact that these bristles still continue to project, 

 while the nematocyst is completely closed (_/?r/. 88, A). They may have been 

 formed originally under the influence of the nematocysts, and then retained the 

 form imposed upon them ; but tlmt partakes too much of the mechanical in its 

 method, and makes them appear as if the mere accidents of contiguity. Moreover, 

 if they were so formed, they ought to stand directly in the way of the shaft of the 

 nematocyst when it protrudes ; but they do not, and so far from it that each bristle 

 stands more or less to one side of the shaft when that is out, and sometimes, even, 

 at a greater distance when the same is retracted. These facts may be more easily 

 elucidated in common fresh-water Ilijdrai than in Lucemarice. We are, tlierefore, 

 rather disposed to think that they were developed, just as the parts of the thread 

 of the nematocysts were, in place. Their connection with the prehensile organs, 

 iunctionally considered, it is true, is inferred mainly from their occurrence only 

 where these organs exist, but we think that that is enough to show at least their 

 architectural relations. Their one-sided, asymmetrical form, added to their lateral 

 position, suggests strongly that they possibly may perform the ofiice of valves or 

 luls ; but on the whole we believe them to be tactile hodies, standing sentinel at 



