40 ANIMALS OF THE SEASHORE 



species are usually very difficult to determine and 

 the identifications are based upon the size, shape and 

 position of the reproductive polyps (gonosomes) and 

 upon the structure of the nutritive polyp or hy- 

 dranth. Some hydranths are uncovered (Tubulari- 

 idae) while most are protected by a horny receptacle 

 or hydrotheca. 



Hydroids are usually attached to some solid 

 object by a root-like structure (hydrorhiza), 

 although a few float freely on the surface of the sea. 

 From this root arises the main stalk, which is usually 

 branched and from these branches hang the various 

 polyps. 



The medusa stage resembles a tiny umbrella. 

 At the lower part of the handle (manubrium) is the 

 mouth which opens into various canals. A mem- 

 braneous velum or veil usually projects inward from 

 the rim of the umbrella. 



The average collector along the New Jersey 

 coast will find only three conspicuous hydroids — 

 Tubidaria crocea and II ydr actinia echinata and Thumria 

 argentea. Most of the other species are small and in 

 order to identify them it is necessary to make a 

 careful study of their structure with a fairly high 

 powered microscope. The student especially inter- 

 ested in the Hydrozoa will find them adequately 

 treated in the references cited in the bibliography. 

 For the benefit of the more casual student, condensed 

 descriptions of some of the New Jersey forms are 

 given here. In writing these descriptions free use 

 has been made of the sources cited in the biblio- 

 graphy. 



Tubularian Hydroids 



Hydroids with naked hydranths, that is, without 

 any protective cups (hydrothecae). 



