301- KR. J. E. DUEEDEN ON THE 



length varying from -034 nun. to -Oi mm. They are invariably a little narrower than in 

 the evag'inations, measuring only -006 mm., and are nearly always perfectly straight. 



In the disc stinging-cells are concentrated within the thickened lips, and are similar to 

 the thin-walled representatives in the tentacles. 



The nematocysts within the stomodseal ectoderm include only one representative of the 

 curved columnar variety. They are very uniform in size, measuring "025 mm. 



The mesenterial filaments are somewhat exceptional in being very sparingly supplied 

 with nematocysts, even in the lower region. They are all narrow and straight, and are 

 nearly of the same length as those in the stomodfeum, namely 025 mm. 



jS^o uematoblasts occur anywhere within the endodermal epithelium. 



It is thus clear that practically every region of the polyp possesses its own distinctive 

 nematocyst. Though generally distributed throughout, the thick-walled variety differs 

 slightly either in size or outline, or in botli, ia the different areas where it occurs. 

 Examples intermediate in type are to be found mingled with the others, so that it is 

 only when taken in nvimbers that the distinctive characteristics become of taxonomic 

 value. 



Tentacles. 



As the column-wall is rarely overfolded in preserved polyps, and the tentacles are 

 usually only slightly retracted, the latter are generally seen in their normal relationship 

 in both vertical and transverse sections (PI. 25. fig. -I & PI. 26. fig. 13). A transverse 

 section, such as is represented in fig. 13, demonstrates that the tentacles are outgrowths 

 of both the entoccelic and exocoelic mesenterial chamhers. In some instances, as at 

 ti and ^5, t., and t^, t^.j and ty,^, it appears as if two tentacles originated side by side from 

 a single mesenterial chamber, but on more close examination a pair of rudimentary 

 mesenteries (III.) can always be detected, so that one of the tentacles is eutocoelic and 

 the other exoccjelic. Such conditions prove that the appearance of a new mesenterial 

 pair is followed very closely by the outgrow^th of a tentacle from its entocoele. In the 

 polyp from which fig. 13 was taken no exotentacles had as yet appeared between the 

 entocoelic members ^5 and i,-, t^ and t-, ^n and iyj. For a short period, therefore, during 

 the s^rowth of new mesenteries, the number of tentacles mav be less than the number of 



O t' 



mesenteries, due to the lagging behind of the exotentacles ; but in the end the sum is 

 the same, a tentacle arising from each chamber, whether entocoelic or exoccelic. 



The tentacular ectoderm and endoderm are both comparatively broad layers, but the 

 mesogloea is very narrow. In sections the wall is about '055 mm. across. The ectoderm 

 is usually thrown into minute rounded ridges separated hy deep narrow grooves, the 

 cells being elongated in the former and very short between (fig. 3). Nematocysts are 

 distributed in a peripheral zone with approximate uniformity throughout the length of 

 the tentacle, and are mainly of one kind : thin-walled, with the close spiral thread 

 showing distinctly, and broad at one end and narrow at the other (fig. 8 a). Numerous 

 examples of the larger, thick-walled variety, such as occurs in the evaginations, are also 

 present, but are slightly shorter and much narrower. 



