Hydractinia, Parkeria, and Stromatopora, 47 



are filled up with structureless sarcode to complete the mem- 

 brane. After this, chitinous points (the " horn-cells/' see 

 'Annals,' I. c.) make their appearance irrespectively through- 

 out the membrane so constituted ; and these sending out pro- 

 cesses more or less sexradiately, which unite with each 

 other, thus form, with additionally superimposed laminae, the 

 chitinous reticulation of which the skeleton of Hydractinia 

 echinata is finally composed (Ann. & Mag. Nat. Hist. I. c. 

 pi. i. fig. 6). When the reticulation has been thus commenced 

 on the first or basal lamina (PI. VIII. fig. 1, «), the upper arms 

 of the sexradiate points or " horn-cells " respectively, which 

 are now free, grow into short conical serrated spines (fig. 1, e, e); 

 and thus the surface of the Hydractinia presents an area of 

 such spines, with minute but variable intervals between them, 

 interrupted only here and there by much larger ones of a 

 similar form (fig. l^g). 



The same process takes place during the evolution of a 

 second or superimposed lamina (fig. 1, c) ,• but here for the 

 most part the descending arms of the " horn-cells " respec- 

 tively unite with the conical serrated or ascending ones of tlie 

 first lamina ; while the opposite or free arms respectively 

 again assume the short conical form, to remain free, or unite in 

 like manner with the descending arms of a third lamina 



(fig. I,/)- 



We have now three lamina (fig. 1, a, c,/), and therefore two 

 intervals or interlaminal spaces (fig. 1, h, d), beyond which 

 the chitinous skeleton o^ Hydractinia echinata seldom extends. 

 In both instances the two intervals are converted into pillared 

 cavities respectively by the union of the ascending and descend- 

 ing arms of the horn-cells respectively ; but the upper interval 

 is much wider than the lower one, and therefore the reticular 

 spaces thus formed much larger. 



On examining the surface of each lamina separately, it may 

 be further observed that many of the short conical serrated 

 spines of the first lamina are not met by cori-esponding 

 descending points of the second one, and therefore remain ^ree 

 (fig. 1, e, e) in the lo^yer interval. This does not appear so 

 often in the upper interval, while, of course, on the surface of 

 the third or last lamina, which is that of the surface of the 

 skeleton of the Hydractinia itself, they are all free (fig. I,/"). 

 Although difiering slightly in height, they average about -^^ 

 inch, which is twenty times less than that of the large spines 

 (fig. 1,^), to which I have above alluded; but while they 

 consist, for the most part, of solid points respectively, the 

 structure of the large spines is more or less reticular, as will 

 now be particularly explained. 



