THE ANATOMY OP ALOYONIUM DIGITATQM. 363 



confined to the basal portions of the secondary polyps and 

 the adjacent portions of their neighbours (fig. 8). 



In addition to these canals^ the mesogloea contains solid 

 rods or rows of cells, isolated cells and spicules (fig. 26, &c.). 



The skeleton of Alcyonium is entirely composed of isolated 

 spicules of calcium carbonate. 



The spicules occur only in the mesogloea, and are far more 

 numerous in the periphery, where they are densely crowded, 

 than in the deeper parts of the colony. 



When an expanded polyp is examined with the microscope, 

 a group of spicules may be seen to extend some little distance 

 up the walls of the base of the extensible portion between the 

 mesenteries (fig. 6). In some polyps a few scattered spicules 

 may extend right up to the slight constriction just below the 

 base of the crown of tentacles. Immediately above this con- 

 striction there is almost invariably found a ring of scattered 

 spicules, which sends a radiating row a certain distance along 

 the aboral side of the tentacles. 



When the crown is examined from above a ring of spicules 

 may be seen surrounding the mouth, and these send off lines 

 of scattered spicules along the sides of the oral surface of each 

 tentacle (fig. 10). 



Section IV. — Minute Anatomy. 



The ectoderm of the general surface of the colony of 

 Alcyonium is very liable to become detached, either by friction 

 in the bottle in which the animal is preserved or during the 

 process of decalcification, and it is only in sections of speci- 

 mens that have been very carefully treated that the ectoderm 

 can be seen at all. 



It consists of a number of columnar, spindle-shaped and 

 flask-shaped cells, 0-02 mm. long, connected at their outer 

 borders, but free from one another for the greater part of their 

 course. At the base of the epithelium there are a few spherical 

 interstitial cells of different sizes, and here and there may be 

 seen a cell which, like a ganglion cell (fig. 17), is irregularly 

 star-shaped. 



