THE STRUCTURE OF XENIA HICKSONI. 195 



processes from the ectoderm cells of these portions, siifficiently 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 ccelentera. 



Besides the extraordinary number of spicules present in the ectoderm 

 and mesogicea of the last few millimetres of the base of the colony, this 

 part is further strengthened by much of the mesogicea becoming con- 

 verted into a dense and horny substance, so that the part of the 

 colony attached to the rock is hard and quite different to the touch 

 from any other part of the colony. This hard base would afford a 

 firm attachment and a rigid support to the branches wmich arise 

 from it. 



The spicules are formed within cells which at first lie in the deeper 

 parts of the ectoderm or have migrated into the outer parts of the 

 mesogicea (Fig. 16). The arrangement of the spicules is not very 

 regular, as they appear to present to the free surface their edge or 

 their flat face indifferently. In nearly all preparations the nucleus and 

 remains of the protoplasm of the spicule-forming cell can be seen. The 

 spicules, which have a horny consistency, have an organic basis impreg- 

 nated with only a very small amount of calcareous matter. They stain 

 deeply with heematoxylin, especially with heemalum. They do not dis- 

 solve when treated with acids, but shrink very slightly, probably 

 owing to the solution and extraction of the small amount of calcareous 

 matter which they contain. They do not offer any difficulties in 

 section-cutting, as the razor cuts through them with little resistance, 

 and sections 2 jj, to 4 /u. in thickness may be readily obtained, although 

 the specimen has not been previously decalcified. On this account 

 Xenia offers exceptional facilities for the study of the development of 

 spicules. As mentioned above, each spicule is formed within a cell 

 lying in the deeper part of the ectoderm. When the young spicule 

 first makes its appearance in the cell its substance is scarcely dis- 

 tinguishable from the protoplasm of the cell ; in fact, it is not until 

 the spicule has attained a diameter of 3 fi to 4 ft that it is possible to 

 clearly differentiate it (PI. XX, Fig. 15). The young spicule is then a 

 small disc which stains with hsematoxylin like the protoplasm of the 

 cell, but its homogeneous structure enables the observer to distinguish 

 it from the finely granular cell-protoplasm which surrounds it. From 



