226 THE AMERICAN NATURALIST. [VoL. XXXII. 
specimens may be examined under a high power, and by a 
proper management of light the cell boundaries, muscle, and 
indeed almost every histological detail may be distinctly seen 
and the movements followed. It was while making such ex- 
aminations of living sarcostyles at Naples in 1895 that I found 
an unexpected proof that the axis of the sarcostyle is not a 
solid rod, but a delicate collapsible tube, the cavity of which is 
strictly homologous with the body cavity of the hydranth. 
While studying a living sarcostyle under a 4y oil-immersion 
lens, the endodermal axis, as it is called, was very sharply 
defined, being separated from the ectodermal layer by the 
“ Stutzlamelle.” Much to my surprise, I saw an amceboid cell 
pass quickly along the exact center of the axis. The cell was 
largely composed of highly refractive granules and exhibited 
` very active amoeboid movements, sending forth well-marked 
pseudopodia and constantly changing form. This mysterious 
cell appeared to be engaged in traveling back and forth between 
the distal and proximal end of the axial cavity of the sarcostyle. 
Its progress was unimpeded and completely demonstrated to 
my mind the presence of an axial cavity in the sarcostyle. 
After having once seen this cell, I looked for them in other 
sarcostyles and found them in nearly every one examined. The 
species under observation was Aglaophenia helleri. I after- 
ward found similar cells in the endoderm of various parts of 
the plumularian colony, particularly in the stem. In such 
localities, however, they did not move from place to place, but 
nevertheless sent forth numerous pseudopodia and exhibited 
amoeboid change of form. 
This demonstration of an axial cavity in the sarcostyle is of 
considerable interest, in view of the fact that it furnishes the 
last and much-desired link in the evidence needed to demon- 
strate the homology of the sarcostyle. It can no longer be 
doubted, it seems to me, that the sarcostyle is the homologue 
of the hydranth; that it is, in fact, a true “person” of the 
hydroid colony, being composed of ectoderm, “ Stutzlamelle,” 
endoderm, and body cavity. It lacks only tentacles to make 
it a hydranth, but we know that certain hydroids, e.g., Proto- 
hydra, have undoubted hydranths without tentacles. 
