232 



HYDROZOA. 



of the colony has at one time formed the external surface, the ex- 

 terior of each exhibits astrorhizse, as a rule, when the specimen is 

 broken open. Very commonly, also, the astrorhizae are placed in 

 successive groups one above the other, those of each vertical system 

 being united by a wall-less axial tube, which usually opens on the 

 outer surface by an aperture, which is often placed at the summit of 

 a conical or rounded prominence. The astrorhizal canals of the 



Stromatoporoids may 

 be compared with the 

 peculiar grooves seen 

 on the external surface 

 of the crusts of the 

 recent Hydractinia, or 

 with the branching 

 and inosculating ccen- 

 osarcal canals of Mil- 

 Iefiora, and, like the 

 latter, they probably 

 transmitted stolons of 

 the ccenosarc. 



The Stromatopo- 

 roids may be divided 

 into two great groups, 

 the forms of one of 

 these having a resem- 

 blance to Hydractiiiia 

 in the structure of the 

 skeleton, while those 

 M*vf of the other possess 

 a ccenosteum con- 

 structed more nearly 

 after the type of that 

 of Milkpora. In the 

 first of these primary 

 sections the skeleton 

 (as in Acti?io stroma, 

 fig. 115, and Labechia, 

 fig. 116) is composed 

 of " radial pillars " 

 and " concentric laminae," which remain definitely recognisable and 

 do not become fused with one another into a continuous reticulation. 

 The type of this section is the genus Acti?iostroma, in which the 

 ccenosteum is massive or laminar, often of large size, and the 

 radial pillars are long and are continued without a break through 

 numerous successive laminae (fig. 115, b). The concentric laminae 



Fig. 116. — a, Tangential section of the coenosteum of La- 

 bechia conferta, from the Wenlock Limestone (Silurian), 

 enlarged twelve times. The section shows the transversely- 

 divided radial pillars {/>), with their axial canals, and their con- 

 necting processes (c). B, Vertical section _ of the same, simi- 

 larly enlarged, the letters as before. (Original.) 



