LUCERNARI.'E AND THEIR ALLIES. 117 



cular disk {figs. 118, 120, «). Tlie ncmatocysts (fi;/. 100, 1) also occupy the whole 

 thickness of tlie gastrophnigma. As to the vibratile cilia (fiij. 100, co) we have 

 only to add that they are unusually long, and very thickly set together. 



j § 30. The Prehensile Cysts. {Nematocysts and CoUeiocysts.) 



215. The Nematocysts (PI. xi, figs. 133 to 145). — These bodies have been so 

 frequently described and so fully illustrated of late years that we do not expect, 

 here, to add materially to the knowledge of them, as far as their general structure 

 is concerned. We merely present some new forms, or at least new variations of the 

 type, from a hitherto unknown source. In an article, which we published under 

 the title of '•'■ Lncernaria the Cceiiotype of Acalephce" in the American Journal of 

 Science and Arts for May, 1863, p. 346, there is a note appended, on p. 352, which 

 is devoted to a concise description of the two kinds of nematocysts that are com- 

 mon to Halidystus auriciiJa. We propose to reproduce that note here, with some 

 few additional remarks, and a number of illustrations. Tliat nematocysts have an 

 iuter-cellular and not an /Mi'ra-cellular position we think has been made sufficiently 

 manifest when describing the cellular structure of the outer wall (opsophragma) 

 of the globular tip (neniatocystophore) of a tentacle (see ^ 203, 203 (A)). We 

 have recorded our opinion elsewhere (Proc. Boston Soc. Nat. Hist., Sept. 16, 1863, 

 p. 283, in note) that they also have an inter-cellular origin, and do not develop 

 within the cells which form the layer in which they are imbedded, but commence 

 their career, de novo, by free cell formation in the cytoblastema. Their relation to 

 the cytoblastema is curiously illustrated in another way ; we refer to the supposed 

 " tactile bristles," which have been described quite fully enough, probably, in a 

 former paragraph (^ 203 (A)). Their presence there, and nowhere else, seems 

 to show a more intimate relationship than usual between the cytoblastema and 

 the cells ; and points argumentatively to the agency of the same cytoblastema in 

 that shaping out of the nematocysts. 



216. A nematocyst of the larger kind (figs. 133 to 141), belonging to tlie tenta- 

 cles, consists of an oval, thick-walled vesicle (ffg. 134, cj), about ttctVo °f ^'^ ^"*^'^ 

 long, or a little less, one end of which is introverted, and projects, in the form 

 of a stout hollow shaft (si), along the axis of the cell, about fonr-fiftlis of its 

 length, and then, rather suddenly thinning into a slender thread (tl), which also 

 is hollow, it bends upon itself, returns nearly to the aperture (7nl) of the 

 cell, and again receding and pressing closely against the inner fiice of the cell- 

 Avall, it forms there a close coil (Jr), which terminates at the end opposite the 

 mouth (ml) of the introversion. In a younger and smaller cyst {fig. 133) 

 tlie shaft (sJ) extends from the aperture almost to the opposite end, and then the 

 thread proceeds to coil up as in the fully grown. The spiral ridges apparently 

 on the shaft of young specimens are formed by rows of bristle-like bodies which 

 are packed closely together, and overlap in the hollow of this introverted body. 

 In an old cyst the separate bristles may be seen in place, while the cell is closed 

 (fig. 134), but it requires very careful research. 



When the coil of thread is ejected (fig. 139, tl), which is accomplished by 



