THE STRUCTURE OF ALCYONIDIUM GELATINOSUM. 31 
chemical composition, and finds them to consist of the 
carbonates of lime and magnesia, largely intermixed with 
organic matter. 
The spicules could not be the result of post-mortem 
changes, for they can easily be seen in the living animal. 
As to their origin and significance, three theories suggest 
themselves. 
They may be accidental, 7.c. foreign matters enclosed 
by the animal. Secondly, they may be developed in the 
ectocyst, to bind together and give rigidity to the gela- 
tinous mass; or, thirdly, they may be vestiges of calcareous 
ectocysts owned by the ancestors of Alcyonidium. 
Against the first theory, it may be urged that they are 
more regular in shape than would be otherwise expected. 
Again, they are calcareous, and the organism would 
probably have taken in grains of sand, which are frequently 
found in the alimentary canal. The forms yielding the 
spicules are from localities far apart, and yet the spicules 
are practically indistinguishable, and there are no organ- 
isms living on our shores which contain anything likely to 
be the producers of the spicules. 
The second theory is more plausible, and there are 
usually numerous active ectoderm cells surrounding the 
spicules. | 
But the third theory seems the most probable, and 
regarding these spicules as vestiges of ancestral structure, 
it would indicate that the Ctenostomatous forms are 
derived from others which possessed a calcareous ectocyst. 
Turning now to the individual polypide, we find it is 
enclosed in a bag or ectocyst. In transverse section the 
ectoderm can be seen extending as a thin, apparently 
structureless layer from the exterior round the cell. 
Lining this is a clear layer of mesoderm, to which the 
