4Q LUCERNARI^ AND THEIK ALLIES. 



It is a vory noticeable fact that the tentacles on the distal side of a group are 

 not tlH- longest, nor, on the whole, the largest; and it is all the more remarkable 

 and uucxnected because some of them are the oldest, and usually appear to be the 

 stoutest This difference is most observable when a file is brought nito profile 

 (cut 2, p. 43) ; then the second, third, and fourth appear to be longer than the 

 outermost one,' and sometimes the third the longest of all. 



94. The form ( figs. 42, 54) of the tentacles in Halicb/stKs, and we have good 

 reason to believe 'in all Lucernaria?, is unique among Acalephse. They do not 

 taper. In the young this seems not to be so, strictly speaking, but that is when 

 they are in a formative state ; and even then we do not recognize the distinct taper 

 wliich is so marked in the globe-tipped tentacles of Coryne, Pennaria, etc. When 

 fully extended a tentacle arises from its base abruptly, with the proportions and 

 form of a very slender cylinder (Jig. 42), from twenty-four to twenty-eight times 

 longer than thick, and terminates in a depressed globose mass (<|)^), the diameter 

 of which is from two to two and a half times greater than that of the shaft {<p'). 



In very old specimens the globe (fg. 41) very commonly has three times the 

 diameter of the cylinder, but from this downward, among the younger members of 

 the group, the difference gradually diminishes {Jig. 34) until it disappears, and the 

 end of the shaft is merely rounded {Jig. 54, A). A group usually presents more or 

 less of a bristling look, as if the tentacles were rigid and inflexible. This is owing 

 to the manner in which they comport themselves with reference to each other ; 

 always free from entanglements, notwithstanding their large number, and rising, 

 with an abrupt, clear stretch, pcu'pendicular from their arched foundation. Fre- 

 quently the rigidity of a group is relieved by a graceful curving of the shafts all in 

 one direction, as if by common consent, and again, without any apparent cause, 

 they assume an angular port, the shaft being bent more or less abruptly, at one or 

 two points, into a zigzag form. Occasionally we may see a tentacle undulated {fig. 

 36.) from tip to base by numerous closely succeeding curves, wliich alternate on 

 opposite sides with such regularity that they have the appearance of being the turns 

 of a helix. Tlie longitudinally ribbed surface is not real, but arises from the con- 

 spicuity of the bundles of muscular fibres which lie just beneath the outer wall. 

 There is evidently such a limit to the extensibility of a tentacle that it can never 

 assume the proporti(jn of a mere thread, such as we see in Hydra, and the scypho- 

 stoma of the Strobiloida ; and we are indeed inclined to believe that it is more re- 

 stricted in this sense than in most Acalephse. The form never passes beyond the 

 proportions of a very slender cylinder, not even approximating a filament in the rela- 

 tions of its length to its breadth. By designating it as a slender shaft we express its 

 relative dimensions in the best general terms obtainable. Its positive length is from 

 one-sixth to one-fifth of an inch when the umbella measures one inch across. Tlie 

 contractility of these organs is also subjected to certain recognizable limits. When 

 disturbed they usually retract so as to be quite thick in proportion to their length ; 

 and if the irritation is persisted in they may become still farther shortened, until 

 the length is to the diameter as four or five is to one. The spheroidal tips do not 

 seem to partake in the general contraction to any appreciable extent. The instinct 

 of self-preservation appears to be exhibited by the consentaneous contraction and 



