444 ON TilE FAUNA AND BOTTOM-DEPOSITS NEAR THE 30-FM. LINE 



and of the nature of the objects to which they fix, several kinds of 

 hydroids may be distinguished, namely : — 



1. Hydroids which usually fix directly to the bottom-deposit. 



a. Those which attach themselves, generally by means of a 

 creeping stolon, most frequently to rocks or stones, less 

 frequently to dead shells. 



h. Those which fix themselves in sand or gravel by means of 

 a felt-work of root-fibres. 



2. Hydroids which usually fix themselves to other living organisms 

 or their remains. 



c. Those which fix to moving organisms. 



d. Those which fix to stationary organisms or remains of 



organisms. 



It must not, of course, be supposed that any species of hydroid is 

 entirely confined to one only of these manners of fixing, but it will 

 be found that for each species there is one (or in some cases possibly 

 two) of these ways of attachment to which its structure is specially 

 adapted, and that in the great majority of instances, more particularly 

 where the species is exceptionally abundant, it will be fixed in this way. 



a. Hydroids attaching tlicmselves to rocks or stones. The nature of 

 the rock is of importance in this connection, though the amount of 

 evidence at present available on this point is not sufficient to render 

 profitable a detailed discussion of the matter. 



Of the hydroids considered in the present paper, those most typical 

 of this class are Garveia nutans, Tubidaria i7idivisa* Aglaoplienia 

 ttihidifera, and Hydrallmania falcata, the latter species being most 

 frequently attached to stones or gravel. Such hydroids are plentiful on 

 rocky or stony grounds. 



h. Hydroids fixing themselves in sand or gravel by a felt-work of 

 root-fibres. Examples of this class are Aglaophenia myrioiihyllum, 

 Antennidaria antennina, and Antennularia ramosa. The texture of 

 the bottom -deposit is a most important factor in determining the 

 distribution of such forms, as will be seen especially by referring to the 

 account of the distribution of Aglaophenia myriophyllum. Antennularia 

 antennina and A. ramosa are perhaps not such good representatives 

 of the class as the species first mentioned, since they are also rather 

 frequently found attached to shells, especially in shallow water. 



c. Hydroids ujhick fix to moving organisms. Hydractinia echinata, 

 Heterocordyle Conybeari, Perigonimus rcpens represent this class. The 

 habit has evidently been acquired by the hydroid by reason of the 

 advantages in the matter of food -supply offered by the constant 

 movement of the host. 



• Cy. p. 448. 



