ECHINODERMATA. 107 



which are filled with sea-water from without, and are believed 

 to exercise a respiratory function, hence the name of " respi- 

 ratory tree " often applied to them. 



The ordinary species of Holothurians, as already said, are 

 all rare, and are mostly only to be obtained by dredging in 

 tolerably deep water. Some of the tropical forms attain a 

 large size, and some are largely searched after to be sold in the 

 Chinese market, being regarded in that country as a delicacy. 



