122 MARINE INVERTEBRATES 
in life and examined with a glass. Some species are confined to 
deep water, but many are littoral and to be found in tide-pools, 
in the chinks and crannies of rocks, under stones, and under the 
hanging Fucus. The horny skeletons of large varieties are fre- 
quently washed ashore, and in their tangled masses smaller 
living species often may be found. 
THE TUBULARIANS 
This division is characterized by zodids borne on long, slender 
stems which are sometimes simple and small, sometimes branch- 
ing and eight to ten inches long. The zodid has two rows of 
tentacles, the central one being sometimes on a kind of probos- 
cis. The reproductive zodids are in bunches, sometimes below 
the outer row of tentacles, sometimes between the two rows. 
The perisare does not cover the zodid. In color they are com- 
monly red or yellow. 
GENus Clava 
C. leptostyla. This species is found growing on Fucus, on the under 
side of stones at low-water mark, and in tide-pools, where it often covers 
several feet of the surface of the rock with a delicate velvet-like carpet. 
It is red in color and is, apparently, a soft and tender species, but it 
thrives on the most exposed beaches. The colonies are cylindrical tubes 
about one quarter of an inch in height, rising from a creeping stem 
(hydrorhiza). Hach tube is surmounted by a zodid with fifteen to thirty 
tentacles, which is constantly changing form by its contractions. Below 
the tentacles are reproductive buds arranged in clusters. Common from 
Long Island Sound northward. (Plate XLI.) 
Genus Hydractinia 
H. polyclina. The soft, pinkish covering often seen on shells inhab- 
ited by hermit-crabs. This association of two different kinds of animals 
is known as commensalism, and is a partnership formed for the benefit of 
one or both the individuals. In this case the mossy appearance of the 
hydroid conceals the shell, while the stinging-cells with which it is in- 
vested are weapons of defense against the enemies of the crab and also 
help to paralyze its prey. In return for these favors the colony is 
moved about, thereby obtaining perhaps better oxygenation. Originally 
it was thought that Hydractinia lived only on the shells occupied by 
hermit-crabs, and that the nomadic life was essential to its existence ; but 
this is not the case, for it is also found growing on rocks in tide-pools. 
These colonies arise from a creeping stem, which forms a horny, root- 
. 
