94 



LUCERNARI.E A iN D THEIR ALLIES. 



broad. Tlieir true, exterior boundary is irregularly polygonal, but rather incon- 

 spicuous on account of the abundance of the pigment masses thereabout ; whereas 

 the interior boundary, which is an irregular oval, or sometimes obscurely poly- 

 gonal, is very sharply and distinctly marked, and moreover so conspicuous as to 

 give the stratum, under a low magnifying power, the appearance of being a mass 

 of dark, granular blastema, filled with clear vacuoles. It will be seen from tliis 

 that the exterior and interior faces of the cell-membrane (d) are not parallel, in 

 other words, that this membrane is of unequal thickness ; and, from the nature of 

 things, incrassated most at the angles. On the whole it is quite thick, but yet 

 plastic to a considerable degree, both in reference to lateral pressure and to what- 

 ever tends to increase its depth at the expense of its breadth. The extensibility 

 of this layer, however, is not altogether a property of the cells, for the interstitial 

 substance is not to be overlooked, nor have we any reason to believe that it is less 

 plastic or dilatable than the vesicles. Nor, on the other hand, do we admit that 

 either the one or the other is capable of extending or retracting itself, unless we 

 altogether ignore the muscular layer {opsomyoplax, fi(j. 85, vi), which presses closely 

 against the inner face of this wall. The contents of a cell are of two kinds, and 

 very simple in quality so far as their mechanical, or rather physical, nature is con- 

 cerned ; tlie one is a perfectly homogeneous, fluid-like, colorless, slightly refracting 

 mass (J-), filling the entire inclosed space, Avith the exception of that occupied by 

 the nucleus, which is the other part of the contents. The nucleus ((?') is the most 

 noteworthy feature of the cell on account of its invariable lateral position. With- 

 out an exception we have, always found it attached to the cell-membrane between 

 the exterior and inner ends of the cell, *. e., against the faces of mutual contact. 

 Here we should naturally expect to find it, in a Avail composed of a single stratum 

 of cells, which grows by self-division of its components, the nucleus, as is well 

 understood, lying in the line of separation. This uniformity of position is carried 

 still further, for the nucleus seems to be fixed about midway between the two 

 extremes of the cell, so that in a profile view {fig. 85) of the stratum they 

 appear as if arranged in a row when observed collectively. We sliall see presently 

 that this arrangement is still more striking in the corresponding wall of the tenta- 

 cles. The outline of the r.uclcus is tolerably well defined, but not sliarp nor rigid^ 

 and the whole presents a uniform, quite conspicuous, colorless, semi-transparent, 

 lenticular mass, attached by one of its broad sides to the cell-membrane. On an 

 average its diameter is about one-fifth or one-sixth the depth of a cell. The inter- 

 cellular blastema not only lies between the cells and beneath them, but completely 

 covers their outer, free ends, and therefore it may be said, without exaggeration, 

 tliat the cells are completely imbedded in the blastema. The pigment (e"), which 

 gives color to this formless mass, consists of irregular granules of great diversity in 

 size, some equalling the nuclei of the cells, and others measuring less, and so on 

 down to tliose wliich are, comparatively, mere specks. The larger granules usually 

 alternate witli tlie cells, filling the interspaces at the angles with a highly refract- 

 ing, often brilliantly colored mass. The finer and speck-like granules crowd the 

 blastema which overlies the ends of the cells, and lend considcral)le aid in impart- 

 ing color to the wall, in fact giving uniformity to the tint, which otherwise would 



