VEEMES AND MOLLUSCA OF THE DEEP SEA 111 



rowing in the sand or perforating rocks ; the Polyzoa 

 are minute creatures that frequently build up by bud- 

 ding large colonies which assume in some cases den- 

 dritic forms like corals, and the Brachiopoda are pro- 

 tected by thick bivalve shells simulating in a striking 

 manner the shells of the Lamellibranchiate mollusca. 



But external form is not the only character that 

 can be relied upon for purposes of classification. The 

 general and minute anatomy, together with the story 

 of the development of these animals, teach us that 

 they are in some way closely related. 



It is not within the scope of this book to enter 

 into the discussion of what these relations are ; suffice 

 it to say that the controversy has within recent years 

 to a great extent turned upon the position in our 

 classification of three interesting genera. These are 

 Phoronis, Rhabdopleura and Cephalodiscus. 



Phoronis occurs only in shallow water, Rhabdo- 

 pleura has been found in water from 40 to 200 

 fathoms deep off the Shetlands and on the Norwegian 

 coasts, while Cephalodiscus was discovered by the 

 1 Challenger ' at a depth of 245 fathoms off Magellan 

 Straits. 



Rhabdopleura forms colonies consisting of 

 branched tubes growing upon the tests of Ascidians, 



