192 POPULAR HISTORY OF THE AQUARIUM. 



It is thus strained before it is used, and leaves the muddy 

 sediment which imparts the peculiar colour which we ob- 

 serve. Underneath this false back in an ample cavity, at 

 the bottom of which is the true dorsal skin. Upon this we 

 find two rows of overlapping plates, which are the breathing 

 apparatus ; these plates being alternately elevated and de- 

 pressed. When elevated, water comes through the felt and 

 fills the cavity ; when depressed, it is expelled at the poste- 

 rior groove, from which intermittent currents may be seen 

 to flow. 



Plate I. contains, — fig. 1. a group of Serjmla contorkipli- 

 catay showing the varieties in colouring of the fans. Pig. 2 

 is the curiously constructed tube of Terehella ; and fig. 3 

 is the worm out of its tube, from a specimen in Mr. Lloyd's 

 collection. 



