326 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
Much remains to he done before we can discuss with profit the economic bearings 
of the subject of this work. It is well known, however, that many fishes feed more 
or less extensively on hydroids. Dr. Edwin Linton has several times called my 
attention to the fact that he often finds hydroids in the stomachs of fishes while 
examining them for parasites. I am inclined to think, however, that hydroids do 
not constitute a very important item in the dietary of our food-fishes, and am rather 
of the opinion that the economic importance of hydroids lies in the fact that the 
presence of these forms in quantities in a given region is of value as an indication of 
abundance of food for fishes in the shape of small crustaceans. It is known that 
many kinds of hydroids live very largely on minute Crustacea, and it follows that 
where the hydroids thrive the fishes will also find an abundant food supply, especially 
in the earlier stages of their development. 
The author is indebted to many naturalists for material that he failed to secure 
himself, and has endeavored to briefly acknowledge these favors in their proper 
connection in the body of the text. 
State University of Iowa, August 10, 1900. 
Key to the families of ITydroida . found in the Woods Hole region. 
A. Hydranths and gonophores not provided with special chitinous receptacles. 
a. Hydranths with scattered filiform tentacles Clavim:. 
o'. Hydranths with a single whorl of filiform tentacles, or two or more closely approximated whorls 
around base of proboscis which might easily be mistaken for a single whorl. 
b. Proboscis conical. 
e. Colony regularly branched Bougainvii.md.e. 
c'. Colony not branched. 
d. Hydrorhizse composed of incrusting, adherent tubules overlaid with a film of ccenosarc. 
Hydractinid;e. 
<V. Hydrorhizse not mutually adherent and not overlaid with a layer of ccenosarc PonocoRYNim®. 
b'. Proboscis trumpet-shaped or hemispherical, the distal portion being the bell of the trumpet or 
equator of the hemisphere Eudendridje. 
a". Hydranths with more than one whorl of filiform tentacles. 
b. A distinct tube of horny perisarc around the stem. 
c. Distal tentacles in two distinct whorls TuBULARiDjE. 
c'. Distal tentacles not in two distinct whorls Hyboconidje. 
b'. No distinct perisarcal tube; stem conspicuously canaliculated; proboscis large Corymorphid.e. 
a'". Hydranths with capitate tentacles only Syncorynid,®. 
a"". Hydranths with a basal row of filiform tentacles, and with capitate tentacles on the proboscis Pennaridje. 
A'. Hydranths and gonophores provided with special chitinous receptacles. (Hydrothecae and gonangia.) 
a. Hydrothecae with distinct pedicel, and with a septum partly dividing the hydrothecal cavity from 
the cavity of the pedicel CampanulariDj®. 
a'. Hydrotheeae with an operculum composed of converging segments Campanulinidje. 
a". Hydrotheeae deep, with pedicels or sessile, and without the septum Lafceid.®. 
a'". Hydrotheeae reduced to saucer-shaped hydrophores ornamented with a necklace of bright dots, 
and much too shallow to accommodate the hydranths Halecid.e. 
a"". Hydrotheeae sessile, and adnate by their sides to the branches on which they are placed. 
b. Hydrotheeae arranged on both sides of the branches , SertularidtE. 
V . Hydrotheeae arranged on one side only of the branches Plumularid.*. 
