260 J. H. ASHWORTH. 



•022 mm. long, "006 mm. to "OlS ram. broad, and "003 mm. to 

 •005 mm. thick. 



Spicules are absent from the tentacles and pinnules of all 

 the polyps examined except one. In the latter a few small 

 spicules (about 8ju to 12^ long) are present in the ectoderm of 

 the tentacles, but only in the proximal "4 mm. 



In the deeper part of the ectoderm, just below the point of 

 attachment of the tentacles, spicules are present in small 

 numbers, while in the middle and proximal portions of the 

 body of the polyp they are very numerous, a tangential 

 section of the body-wall in this region showing that the 

 spicules form an almost complete layer in the deeper part of 

 the ectoderm. Spicules are numerous in the ectoderm of the 

 stem, and especially numerous in the angle of the Y-shaped 

 piece formed at the point of division of a stem. In the lowest 

 parts of the colony, i.e. in the portion attached to the rock, 

 they are present in enormous numbers, practically filling the 

 entire mesogloea in that region. The largest examples of 

 spicules are to be found in the basal portion of the colony. 

 (Those shown in fig. 13 are from this region.) The absence 

 of spicules from the tentacles, pinnules, and from the body of 

 the polyp around the base of the tentacles is correlated with 

 the power of contractility, slight though it is, which is pos- 

 sessed by these parts. The presence of an almost complete 

 layer of spicules forming a more or less rigid cylinder in the 

 middle and proximal portions of the body of the polyp and in 

 the stem, together with the absence of muscle processes from 

 the ectoderm cells of these portions, sufficiently accounts for the 

 non-retractile character of these parts of the colony. The 

 increased number of spicules in the angle between two branches 

 gives the required rigidity to this part, and prevents flexure of 

 the branches and consequent closure of some of their canals 

 and coeleutera. 



Besides the extraordinary number of spicules present in the 

 ectoderm and mesogloea of the last few millimetres of the base 

 of the colony, this part is further strengthened by much of the 

 mesogloea becoming converted into a dense and horny substance. 



