116 INVERTEBRATE. ANIMALS. 
without, and are believed to exercise a respiratory function, 
hence the name of “respiratory tree” often applied to them. 
The British species of Holothurians, as already said, are all 
rare, and are mostly only to be obtained by dredging in ‘toler- 
ably deep water. Some of the foreign 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. 
