small and inconspicuous and seldom recognized as 

 among the most abundant animals of the seashore. 

 The plumelii^e colonies that rise to heights of 6 feet 

 from their moorings on the sea bottom at depths of 

 three thousand feet, and the equally tall solitary 

 polyp, Branchiocerianthiis, which has been dredged 

 up from fifteen thousand feet below the surface, are 

 conversation pieces. The vast majority range from a 

 fraction of an inch high to several inches, with 8 to 

 12 inches as the upper extreme; and they are largely 

 confined to the shores and to shallow waters, where 

 they form delicate white, pink, violet, or brown tufts 

 on rocks and wharf pilings, on seaweeds, and on 

 many animal stalks and shells. 



Hydroids are often mistaken for minute seaweeds 

 because of their plantlike growth forms, though 

 there the resemblance ends. The "flower heads" of 

 solitary polyps or those at the free tips of branching 

 colonies are voracious feeders on minute crustaceans 

 and worms, eggs and larvae, or even on fishes. Few 

 things are more beautiful than a microscope field 

 filled with row upon row of elegantly shaped, glassy 

 hydroid chalices, each with a circle of graceful trans- 

 parent tentacles — or more potentially lethal. .\ small 

 animal swimming through has as much chance of 

 making it safely as a ship would have in sailing 

 through waters so heavily mined that the detonating 

 devices were almost touching. At the slightest alarm 

 the outspread tentacles are whisked in with a liveli- 

 ness that at once betrays their animal nature. 



Favored and protected positions are acquired by 

 hydroids that attach to the branches or stalks of ses- 

 sile animals like sponges, sea whips, or sea pens — 

 or that are constantly carried to new pastures on the 

 shells inhabited by hermit crabs. Many hydroids 

 share in the host's feeding currents by fastening to 

 the outside of moUusk shells or worm tubes near the 

 water intake. Some even live within the mantle cav- 

 ity of certain oysters or other bivalves, and in the 

 sievelike branchial cavity of certain tunicates. 



There are two main types of hydroids, divided ac- 

 cording to the extent of the transparent yet protec- 

 tive horny covering that the polyp or polyp colony 

 secretes about itself. Most familiar of the fully 

 sheathed hydroids is the cosmopolitan genus Obelia, 

 with species on almost all shores. Some form delicate 

 little white sprays that rise 1 or 2 inches above the 

 creeping runners that attach to rocks, seaweeds, mus- 

 sels, wharf pilings, or floating wood. In shallow wa- 

 ters below low-tide mark, yellowish species of Obelia 

 grow 8 to 12 inches long. The various species are 

 alternately branched or have stems that zigzag, with 

 tentacled polyps arising at the angles and lodged in 

 goblet-shaped transparent cups. From the axils 

 formed by the stalks of these feeding polyps arise 

 urn-shaped reproductive polyps enclosed in trans- 

 parent containers of the same shape. At intervals 



these bud off little medusas that swim away. Though 

 they are barely visible to the naked eye, their jerky 

 pulsations are familiar to those who examine plank- 

 ton under the microscope. The muscular shelf that 

 characterizes hydrozoan medusas is aborted during 

 development, but otherwise the Obelia medusa is 

 typical of the little flattened saucers allied to the 

 sheathed hydroids. It has four digestive canals ra- 

 diating from the central digestive cavity to the rim of 

 the umbrella, and to these canals are attached the 

 sex organs. 



Besides the campanularian or bell-like hydroids, 

 to which Obelia belongs, the many kinds of sheathed 

 hydroids include such widespread families as the 

 sertularians, in which the cups are not stalked but 

 are set directly on long, graceful stems that branch 

 to look mosslike or fernlike. A species of Sertularia 

 was referred to earlier as the "'whiteweed" of a small 

 industry in England. In the plumularians the cups 

 are also set directly on the stems, but the branches 



Portion of a hydroid colony, magnified through a 

 microscope, shows the many polyps with outspread 

 stinging tentacles that lie in wait for small passing 

 prey. (Bermuda. Ralph Buchsbaum) 



