Ilydractiiiia, Parkcria, (tiul Stromatopora. 49 



stolons, about -yj-s 'ncii in diameter (fi;^. .3), wliicli licrc and 

 there produces corresponding grooves in the chitinous struc- 

 ture ; while in some parts it is almost free, and at others 

 covered with chitinous points (fig. 3, r), wliich arc in continua- 

 tion with the surface structure of tlie skeleton. This coeno- 

 sareal tuhulation also here and there })reseiits short branches 

 which, from their annnlation (tig. 3, ^/), aj)pear to have been 

 the ])edieels of polynites — a ringed feature which is remark- 

 ably common on tlic stems and pedicels of the llydroid 

 zoophytes, and one to which it is desirable here to direct 

 attention m a sectional point of view, viz. : — 



As the " annnlation " consists of circular constrictions of 

 the stem following each other in a moniliform maimer, so, 

 when a horizontal section is made of this part through the 

 interval between the constrictions, the latter presents the 

 appearance of a circular diaphragm or line with a circular 

 hole in its centre {fig. 3, h) ; and if the section be oblique, 

 then there is a succession of fragmentary circular lines ending 

 in an entire circle, completed by the addition of the cut line 

 at the inner end of the section of the stem to the semicircle of 

 the dia])iiragm, thus altogether somewhat resembling the spiral 

 line of a '' thread-cell " (fig. 23). 



Further, it is desirable, for our present pui-pose, that all the 

 skeleton-structure of Ihjdractinia ecldnata should be borne in 

 mind, while we discard the sarcodic parts, as they may be 

 assumed to be destroyed long before fossilization. 



Hence we should remember that the small spines remain 

 free on the surface of the laminae respectively, and thus retain 

 the conical serrated form as they appear on the surface of the 

 entire skeleton, while other spines arc joined to, incorporated 

 with, and thus support the following lamina ; also, that the 

 large spines are hollow in the body and solid or closed at the 

 summit, while the structure is more or less radiated through- 

 out. 



Nor should the structure of the laminaj be misconceived, 

 inasmuch as, although in a vertical section they give the 

 appearance of a continuous layer, still this chiefly arises from 

 the union of the horizontal arms of the horn-cell, when viewed 

 in the vertical section ; while if viewed horizontally^ they 

 present the reticulation seen on the surface of the skeleton, 

 which is that of each lamina in succession. 



To facilitate an understanding of the way in which the 

 skeleton of Ilydractinia echinata is developed, I have taken 

 the most regularly formed portion, which, as will be seen by 

 my illustration in the 'Annals' {I.e. pi. i. fig. 6), has very 

 much the appearance of that of a hexactinellid sjjonge ; but 



Amni'Maj.N. Hist. Ser. 4. Vol.x'w. 4 



