LUCERNARIyE AND THEIR ALLIES. 119 



scope, it is plain enough that it is solf-contractile. As we are not in the habit of 

 using that dangerous instrument, the covqjrensonum, we have no allowance to make 

 for mistakes in that direction. Whether a thread, after being once ejected, is 

 ever retracted into the cyst again remains as yet undetermined by observation, but 

 tlie circumstances of its eversion render it highly probable that it is not. 



217. The smaller lumatocijsts {frjs. 142, 143, 144, 145) are much more simple 

 in structure than the larger forms, but remarkable in other respects. The intro- 

 verted shaft (Ji[/. 142, si) is very slender, in fact no larger than tlie rest of the 

 thread. It does not project into the axis of the cylindrico-oval cell, but presses 

 close to its side, and extends four-fifths of the way to its opposite end, and then, 

 bending abruptly upon itself, the thread passes with a long curved sweep nearly to 

 the aperture of the cell, whence it again returns with another long sweep, which 

 is repeated eight to ten times {fgs. 143, 144), until the inner foce of the cell-wall 

 is lined by a close coil (/c), which winds lengthwise, instead of transversely, as it 

 does in the larger nematocysts (_/?</. 134). When extended (Jiij. 145, tl) the thread 

 is from twelve to fourteen times the length of the cell ; and it offers not the least 

 sign of appendages of any kind, but is simply a smooth, round filament, of uniform 

 thickness throughout, except at the end, where it tapers slightly and terminates in 

 a blunt tip. The cell itself, when everted, is sensibly diminislied in size, and nar- 

 rows rapidly into the prolonged filamentary portion. Both of these kinds of nem- 

 atocysts, and these only, are found in other parts of the body besides upon the ten- 

 tacles, but they vary a great deal in size, and in some cases are very small, as, for 

 instance, in the adherent disk of the pedicle. Mobius has favored us with a copy 

 of his memoir (Ueber den Bern, etc. etc., der Nesselknpsehi, etc.; Abhandl. des 

 Naturwiss. Vereins zu Hamburg; Erstes Heft, des fiinften Bandes, 1866) upon 

 these bodies, in which he claims that the shaft is closely invaginated upon itself, 

 when the cyst is closed. We have not had an opportunity of looking at this point 

 since the reception of the paper. If it be true, it seems to explain tlie sudden jerk 

 with which the shaft of the ncmatocyst is often everted, as if with explosion. This 

 is remarkable in the cysts of Hydra. 



218. The Colletocysts {fi<js. 96, 97, 101, 102, «, «-).— The discovery of these 

 bodies added a third kind of prehensile cyst to those so well known among the 

 Acalephie and Ctenophorae. Elirenberg recognized the office of tlie nematocyst 

 (Abhandl. Berlin Akad., 1835 [1837]), but we owe to Doyere (Compte Rendu, 

 Aoiit, 1842) the original solution of its structure, as an invaginated, hollow thread 

 within a cyst of which it is a direct prolongation. We, ourselves, were so fortu- 

 nate as to discover the structure of the second, or non-inraginafed type of nema- 

 tocyst peculiar to the Ctenopliorjc (see H. J. C, in Agassiz, Contrib. Nat. Hist. 

 U. S., vol. iii, p. 231, Jigs.). The original sketch of the third kind of cyst, the 

 colldoeyst, was published by us several years ago in tlie paper on the '■^ Cwnotype 

 of Acajephai''' (I'roc. Boston Soc. Nat. Hist., March, 1862; and American Journal 

 of Science, May, 1863). 



In previous paragraphs (^ 110, 208 (A), 117, 115, 212) of this memoir Ave 

 have described the position of these bodies among the cells with which they are 

 associated, on the colletocystophores, digituli, and the adhesive "disk" of (he 



