THK SKRTPLARIDjE. ]',) 



In the examination of serial sections of hydrothecffl I have been unable to tind any epithelial 

 structure of this sort. It is probable, however, that there is at times a chitinoua lining to the 

 hydrotheeal chamber that may be separated from the hydrothecal wall through shrinkage. If 

 we remember the origin of the hydrothcea and its relation to the you MIT hydranth a verv probable 

 explanation suggests itself. The hydrotheea is fornu'd as an excretion from the epidermal cells 

 of the budding hydranth which fills the cavity of the hydrotheca until the latter has attained its 

 full si/e and final form. An examination of the hydrothecal walls under high magnification and 

 in sections shows that they are laminate, as if formed by the deposition of successive thin layers 

 of chitin. When the hydranth nears maturity it withdraws from contact with the hvdrothecal 

 walls, the separation proceeding from below upward, the top of the hydranth being the last to 

 become separated from the hydrotheca. The last area of union, therefore, is annular and near 

 the toj) of the hydrotheea just beneath the margin. Thus it will be seen that the last delicate 

 lay or of chitin would line the hydrotheca up to a ring-like band which may he somewhat thicker 

 than the rest because hero the secreting surface has been longest in contact with the hvdrotheca. 

 Under certain conditions it is entirely conceivable that this thin membrane .should become 

 separated from the hydrothecal wall, of which it is really a part, by shrinkage, especially in 

 preserved specimens. In this case it would be apt to remain attached to the annular area longer 

 than to other portions on the hvdrothecal walls, and we would then have exactly the appearance 

 which attracted the attention of Ilartlaub. The lining is not epithelial, however, in a strict 

 sense, and does not belong properly to any one of the cell layers, because it is not composed of 

 cells or of modified cells. On the contrary, it is hisrologically and morphologically merely the 

 innermost of a number of layers excreted by the ectoderm of the hydranth and forming the 

 hydrothecal walls. 



Tli, ()j>,i-i'iiliiin. This is a structure of great systematic importance that has been made 

 especially prominent through the careful work of Prof. (T. M. H. Levinsen, and his admirable 

 presentation of the results of his investigations. 1 



One of the earliest specific accounts of the operculum among the Sertularidae is found in 

 Louis Agassi/'s Contributions to the Natural History of the United States.- This author, in 

 describing the growth of the hydranth, says : "And the body retracts altogether from the calycle. 

 after having completed the formation of a bivalve-like operculum." "When the hydra pro- 

 trudes from its calycle for the first time it pushes aside the operculum, yet the latter may remain 

 adherent for some time, but evidently for no particular purpose." 



A still earlier mention of the operculum, in the genus S,-rtiil<n-tllii. is found in a work 

 written in 1X57 by Joshua Alder, :l who notes the presence of the four-parted operculum in A. 

 ritt/nKii and .s'. /<//.//</, hut seems not to have observed it in X. y'///.:v////</.v nor in X. h-!<-UKj>!iliitii. 

 both of which species were known to him. In 1SIJX Hincks, in his British Ilydroid Zoophytes, 

 mentions the opercula in the genera X, /-f ///,/,; II, i, DipAostffl, and Tlutiitrin. but does not seem 

 to have found it in X, linlnrin. Allmaii, in his Clm/l, ,u/, r Report, the Ilydroida. Second Part. 

 1888, gives as a part of his definition of Suinl<n-!<t " orifice with or without an operculum" 

 (p. 50), and has this to say regarding the operculum : " The valves in all these cases are so thin 

 and perishable that it is only in recent or exceptionally well-preserved specimens we can hope to 

 meet with them, a fact which in itself deprives the distinctions derived from them of that prac- 

 tical value which ought, if possible, to be found in all well-selected systematic character-" (p. ~>l). 

 This author also adopts a " lid-like operculum formed by a single valve" as a character of the 

 genus I>i]>/i<isi*t, and notes that two species of his genus Desmoscyphus, I>. j>,i-t !n<tt UK and 

 D. acfl>it/x'<U'/>!/x, possess opercula. 



'Oin Fornyt'lsi-ii :if KriwrinirsiiicliviiltTni- Ims II ydrniilcrnt' ; Vi<lriisk;il'li'_r<- Mr,l,lrli-lsrr Ira den iuitiirhi.-tnri>ke. 



i Kjobenhavn., ls!i_; ( '..prnliuirrn, ism'. 

 Mctlust-r Ctenophorer <><; Hyilruiilrr I'm I ;n>nl:iM.l^ Wstk\>t tilliiri-mril IViiiHTkiiiiiircr <mi I Iv't 



ik ; idem., Copenhagen, isici 

 "V..I. IV, isti-j, (1 . :i. 

 'A Catalogue of Zoophytes of Nurtliiiinlierluiiil and Durham, Xi-wraMtlc-nii-Tyni'. Is.'.T, p. -'.'>. 



