SYSTEMATIC POSITION AND AFFINITIES. G7 



In order to satisfactorily compare the skeleton of Hydractinia echinata with, 

 that of a Stromatoporoid, it is best to take the thickened portion of an old colony 

 of H ij clr actinia, where it surrounds the mouth of the invested shell. In this region 

 the shell itself has usually been absorbed, so that decalcification is not needful, 

 and it is easy to make thin sections, both in a vertical and a tangential direction. 

 On looking at the surface (PL VI, figs. 3, 3, a) we see that it is studded with nume- 

 rous small projecting tubercles, which represent the free upper ends of the 

 "radial pillars-." Intermixed with these are numerous larger serrated "spines" 

 (PI. VI, fig. 6), which are apparently formed by the upward growth of a number 

 of the radial pillars, and by the coalescence of the free ends of these into a loose 

 reticulation. Between the bases of the tubercles and spines may be seen minute 

 circular apertures, which either give exit to polypites, or which serve for the 

 passage of stolons which place the superficial layer of the coenosarc in connection 

 with the deeper layers of the same. The surface also exhibits the shallow, irre- 

 gular astrorhizal grooves. The surface-lamina is, therefore, in the main, only a 

 repetition of the " basal lamina," as also of all the lamina? intervening between the 

 first and the last-formed layer. The principal difference is only that the " astro- 

 rhizal" grooves have the form of shallow open gutters on the surface of the last- 

 formed lamina, whereas they are necessarily in all the other lamina? more or less 

 completely roofed over, and converted into canals by the growth of each new layer 

 in turn. 



Vertical sections of the thickened colony (PI. VI, fig. 5) show that it is com- 

 posed of numerous parallel chitinous rods, which are perpendicular to the surface 

 and to the invested base, and which are united at intervals by horizontal horny 

 fibres. The vertical rods are the " radial pillars," produced by the upward growth of 

 the primitive horn-cells ; the connecting fibres are the horizontal " arms," which the 

 pillars give out at intervals ; and the spaces between these are filled with the coeno- 

 sarc, and represent " interlaminar spaces." In tangential sections (PI. VI, fig. 1) 

 we see the cut ends of the transversely-divided radial pillars, in the form of round 

 horny nodes, which are united by the irregular radiating arms which they give out 

 at intervals. Many of the radial pillars run continuously from the basal lamina 

 to the surface, where their free ends project as tubercles or spines ; but others do 

 not seem to be continued through more than two or three successive lamina?. 

 Moreover, each lamina, in turn, may give rise to short ascending tubercles or 

 spines, which simply project into the interlaminar space, but do not reach the next 

 lamina above. 



Upon the whole, it must be admitted that there is a remarkable similarity 

 between the minute structure of the chitinous skeleton of Hydractinia echinata and 

 that of the large calcareous ccenosteum of certain of the Stromatoporoids, more 

 particularly of the genera Actinostroma, Nich., and Labechia, B. and H. 



