300 Mr. G. 0. Bourne. On the Structure and [June 21, 



mnnication between the polyps and their connecting canals on one 

 side of the colony and those on the other side of the colony. 



The polyps have been fully described by Moseley. They are 

 scattered irregularly over the surface, and the only important feature 

 presented by them, in which they differ from other Alcyonaria, is the 

 complete introversion of the tentacles during retraction. Surrounding- 

 each polyp, and occupying all the surface of the colony, are very 

 numerous tubes, ending blindly below, and closed above by the sheet 

 of superficial ectoderm which covers the exterior surface; these are 

 the ccenenchymal caeca. They occupy cavities in the corallum known 

 as ccenenchymal tubes, and are set at right angles to the surface of 

 the colony. The ccenenchymal cceca communicate with one another, 

 and with the polyps, by means of a network of canals, which lies close 

 beneath the surface ; these are the superficial endodermic canals. 



At the growing edges of the colony the superficial network is not 

 well developed, the ccenenchymal coeca are closely contiguous, and 

 open into one another at their outer ends, either directly or by means 

 of short, irregular, transverse passages which cross over the partitions 

 separating adjacent ccenenchymal tubes. 



The cceneuchymal coeca, superficial canals, and polyps are lined 

 internally with endoderm. Outside this is a thin layer of mesoglcea, 

 and outside of this an irregular layer of large, dark- staining, 

 granular, fusiform cells, which are calcigenous, and may be called 

 calicdblasts. 



The calicoblasts were described by Moseley as mesodermic, but 

 they occupy the position of ectoderm, and they are, in fact, derived 

 directly from the superficial sheet of ectoderm. Their origin is seen 

 in sections made perpendicularly to the surface of the colony at its 

 growing point. Here the ectoderm cells are generally elongate and 

 pyriform, their broader outer ends resting on a distinct external 

 limiting membrane, their inner ends tapering and produced into long 

 processes, which may often be traced into connexion with deeper 

 seated fusiform cells, of more granular character. The deeper seated 

 cells are imbedded in a thick, homogeneous, gelatinous substance 

 which lies immediately below the ectoderm, and is the newly formed 

 mesogloea, thicker here than elsewhere in the colony. Study of 

 numerous sections shows that the deeper seated fusiform cells are 

 derivatives of the elongate ectoderm cells, and that some of them are 

 used up in the formation of the mesogloea they appear to dissolve 

 and to be wholly converted into a structureless gelatinous mass 

 whilst others increase in size, develop many refracting grannies in 

 their interior, and become calcigenous calicoblasts. In many places 

 the calicoblasts may be traced into direct connexion with the 

 ectoderm. The coenenchymal cceca of Heliopora do not appear to be 

 degenerate siphonozooids, as was suggested by Moseley, but rather to 



