g2 LUCERNARI.'E AND THEIR ALLIES. 



the very beginning to the latest period of life there is always the same, unvarying, 

 siwjle stratum of cells in the opm^'hrayuat and the yastrophragma, no matter how 

 great the changes they undergo in other respects. 



163. The bud soon becomes a prominent feature by rising quite abruptly from 

 its foundation. It does not, however, present any marked change in its constituent 

 walls, nor in its proportion, except that it becomes distinctly cylindrical, until it is 

 longer than it is broad, and then (Ji;/. 54, A) it begins to expand more rapidly at 

 the\ase and becomes pyriform. At no time does its interior (<|)^) cease to commu- 

 nicate openly with the general cavity of the body; nor is the passage-way into it 

 flanked by intertentacular processes until it has advanced considerably beyond the 

 mere bud-like stage. At the beginning of the pyriform stage {fig. 64, A) the 

 outermost and innermost walls are a great deal thicker near the tip ((|)') than at 

 the base (<^') of the tentacle. The outer wall gradually thickens along the shaft 

 until it becomes twice as deep in the region of the nematocysts as below. In the 

 innermost wall the difference is not so great by one-half, nor is the transition so 

 gradual, the thickening proceeding very rapidly where the opposite faces converge 

 at the tip. The circumscription of the nematocystigerous area is not yot distinct, 

 as we find the nematocysts extending down the shaft, in diminishing numbers. 

 The myoplax and the chondromyoplax begin to be distinguishable as separate 

 layers, but the latter is very thin, a mere film. It will be judged from these facts 

 that the development of a tentacle is not a mere mechanical protrusion of the walls, 

 stretching outward into finger-like processes and there fixing themselves. If it 

 were so the already well developed chondromyoplax of the circumjacent area would 

 be fully represented, and not obscurely merged into the myoplax. There is, how- 

 ever, one remarkable feature to be noted here, and that is, the bud is very nearly 

 as broad as the bases of the oldest tentacles. This is not because the walls are as 

 thick, but the cavity is much broader in proportion. On this account it is not 

 an easy matter to determine the relative ages of those which stand intermediate 

 between the pyriform stage and that in which the globose tip is distinctly marked 

 out. 



164. From tliis time onward the principal changes consist in the elongation of the 

 shaft, and an increasing slenderness ( fig. 34), Avhile its' innermost wall {fg. 48, i) and 

 the chondromyoplax (6') thicken gradually, and diminish the diameter of the cavity. 

 At the same time the nematocystigerous region {^-) increases very rapidly in depth, 

 and its boundaries become insensibly more circumscribed, until by the time the 

 tentacle has attained a length which is from ten to twelve times greater than the 

 diameter of the shaft, it finally assumes a decidly globular form, and is separated 

 from the shaft by a sharply defined line. The latter is particularly noticeable in 

 the largest tentacles {figs. 54, 55) of full grovn individuals, where it may be 

 recognized as a narrow, light band, in reality a sharp constriction, at the line of 

 junction between the shaft ((|)0 and the splieroid {cp-). It is also observable in 

 quite young tentacles {fig. 33, (J)-*), but is far less conspicuous than in the oldest 

 of these organs. The last noticeable change occurs after the tentacle is very 

 nearly full grown, and then the spherical tip becomes depressed at the distal end 

 and assumes a more or less oblate spheroidal figure {figs. 41, 42). 



