TWO SPECIES OF HYDRACTINIA 
473 
any of the specimens examined by me. There is no doubt that 
the chitinous spines of the hydroid have no relation to the spines 
of the gastropod shell, since the latter are usually absent. Again, 
if, as Stechow supposes, the entire colony is developed on the sur- 
face of a gastropod shell which is then dissolved away, the skele- 
ton of the h\^droid ought to form an exact cast of the shell; but 
that the former may depart widely from any form of gastropod 
shell may be seen on examining a few specimens of the hydroid. 
This point is also shown very well in the photographic reproduc- 
tions of Stechow,^^ in which several degrees of spiral winding are 
manifest. 
The outer surface of the hydroid, which is chestnut brown in 
color, is sculptured by numerous fine, irregular, elongated, anas- 
tomosing ridges and furrows, which are especially apparent on 
the surface of the spines, but are completely hidden from view 
where the surface is covered by a thick layer of coenosarc. Their 
appearance and arrangement vary so in different specimens that 
a general description can hardly be given, the anastomoses in 
particular being so numerous in some cases that a fine reticulated 
appearance is brought about. The inner surface of the hydroid 
shell is perfectly smooth, being lined by a very thin subcontinuous 
chitinous lamella which would come in direct contact with the 
body of the hermit crab. The thickness of the chitinous shell 
may amount to more than 1 mm., the spines being left out of 
account. In the body of the shell the chitin is comparatively 
soft, and there is no difficulty in preparing thin free-hand sections 
with ordinary razors. In such a section carried across the whole 
thickness of the shell, it is seen to consist of a dense spongework, 
exactly as in Solanderia (Dendrocoryne, Ceratella, Dehitella, 
Spongocladium, Chitina) (fig. 3). Toward the outer surface 
the meshes are larger, the trabeculae are stouter and darker in 
color, and are seen to form projections and indentations corre- 
sponding to the ridges and furrows on the surface. The meshes 
become smaller and rounder at a short distance from the outer 
surface and remain nearly constant inwards. The inner surface 
^Ujoc. cit.: '09 pi. 1. 
