TWO SPECIES OF HYDRACTINIA 
491 
I shall confine myself to the points on which ni}' own observa- 
tions differ from those of Inaba, but in criticizing his descriptions 
it must be borne in mind that his work was done under some pecu- 
liarly difficult conditions and with insufficient literature at his 
command. 
Fig. 19 A colony with polyps. Xat. size. 
In the first place, what Inaba thought was a worm-tube is really 
the chitinous skeleton of the hydroid. It has the shape of a 
gastropod shell, is much more regular, judging from my specimens, 
than the corresponding structure in H. sodalis (fig. 19), and by far 
the larger part of it is soft and of a dirty greenish hue. The inner 
surface is perfectly smooth, and when examined with a hand lens 
shows exceedingly fine reticulations, due to the inner structure to 
be mentioned later. The thickness of the hydroid shell varies a 
good deal in different parts, being very thin near the mouth but 
becoming as thick as 2 mm. or more near the apex. On the outer 
surface are found much dirt, minute sand grains, diatoms, etc., 
and much of the thickness of the subapical part of the shell 
just mentioned appears to be due to the presence of these extrane- 
ous bodies. The chitin of the skeleton is flexible but very strong, 
so that it is relatively more difficult to obtain good serial sections 
