IIv(lr;ictiiii;i, Paikcria, <7/u/ Stroraatopora. 47 



aro filK'il up with stiiictiiit'U'.ss sarcodc to complete the Tiicin- 

 braiie. After this, chitinous points (the " horn-cells," see 

 'Annals,' /. c.) make their a])pearance irrespectively throuj^h- 

 out the membrane so constituterl ; and these sendin;!^ out pro- 

 cesses more or less sexradiately, which unite with each 

 other, thus form, witji ad<litionally suj)erimj)ose<l lamin.T, the; 

 chitinous reticulation of whitii the skeleton of Ifi/drnctini'ii 

 echinata is finally com})osed (Ann. & iMaji;. Nat. Hist. /. c. 

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

 on the first or basal lamina (PI. VIII. fiii^. 1, a), the upper arms 

 of the sexradiate ])oints or " horn-cells " respectively, which 

 are now free, grow into short conical serrated .spines (fig. 1, <', r>)| 

 and thu>* the surlace of the llijdi-dctinia presents an area of 

 such sj)incs, with minute but variable intervals between them, 

 interrupted only here and thcne by much larger ones of a 

 similar form (fig. 1,,^;'). 



The same process takes ])lace during the evolution of a 

 second or superimposed lamina (fig. l,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 the 

 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 



W e have now three hnuina?. (fig. 1, a, c, /"), and tliercfore two 

 intervals or interlaminal spaces (fig. 1, 6, (/), beyond which 

 the chitinous skeleton oi Ih/ih-actinia 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 resj)ectively ; but tlie up|)er interval 

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

 spaces thus formed much larger. 



On examining the surface of each lamina sc))arately, it may 

 be further observed that many of the short conical serrated 

 spines of the first lamina are not met by corresponding 

 descending points of the second one, and therefore remain //-ee 

 (fig. 1, e, e) in the lower interval. This does not ap])ear so 

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

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

 .skeleton of the Ifydrartinia itself, they are all free (fig. 1,/'). 

 Although differing slightly in height, they average about rj-^„ 

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

 (fig. \,g), to which I have above alluded; but wliile they 

 consist, for the most jiart, of solid ])oints respectively, the 

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

 now be particularly explained. 



