Hjclractinia, Parkena, and Stromatopora. 49 



stolons, about ^^-g-s- iwch in diameter (fig. 3), which here 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, c), which are in continua- 

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

 sarcal tubulation also here and there presents short branches 

 which, from their annulation (fig. 3, a), appear to have been 

 the pedicels of polypites — a ringed feature which is remark- 

 ably common on the stems and pedicels of the Hydroid 

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

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



As the " annulation " consists of circular constrictions of 

 the stem following each other in a moniliform manner, 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, b) ; 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 diaphragm, thus altogether somewhat resembling the spiral 

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



Further, it is desirable, for our present purpose, that all the 

 skeleton-structure of Hydractinia echinata 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 laminse respectively, and thus retain 

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

 entire skeleton, while other spines are joined to, incorporated 

 with, and thus support tlie 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 laminse 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 Hydractinia echinata is developed, I have taken 

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

 my illustration in the ' Annals' (/. c. pi. i. fig. 6), has very 

 much the appearance of that of a hexactinellid sponge ; but 



Ann. & Mag. N. Hist. Ser. 4. Vol. xix. 4 



