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 zooids borne on long, slender 

 stems which are sometimes simple and small, sometimes branch- 

 ing and eight to ten inches long. The zooid has two rows of 

 tentacles, the central one being sometimes on a kind of probos- 

 cis. The reproductive zooids are in bunches, sometimes below 

 the outer row of tentacles, sometimes between the two rows. 

 The perisarc does not cover the zooid. 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). Each tube is surmounted by a zooid 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 Hyclr actinia 



IT. 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- 



