OF RESPIRATION". 



217 



thus the stratum in contact with the vascular membrane 

 is incessantly renewed, and respiration thereby maintained. 

 [In the Echinidce (fig. 174), the space comprised between 

 . iscera and the test is filled with water, which is drawn 

 into and rejected from the body by five pairs of mem- 

 branous respiratory tubes, collected into ten tuft-like organs, 

 situated around the circumference of the oral aperture, and 

 opening internally by two perforated pits, as in Asterias. 

 The water thus introduced into the interior of the test 

 flows along the membrane, covering its surface, and over 

 the peritoneal layer, investing the digestive organs and 

 tubular feet and ovaria by the action of cilia, so that the in- 

 terior of the 



of the 



' is in- 



:ly tra- 

 versed by re- 

 spiratory cur- 

 rents, whilst 

 the blood, 

 circulating 

 through the 

 coriaceous in- 

 tegument, is 

 in like man- 



itedby 



currents flow- 

 ing over its 

 surface by the 

 vibrations of 

 cilia. 



In the Ho- 

 lotluiria (fig. 



the re- 

 tory 

 function is 



1 to a 

 pair of or- 



t'ormed 



a type 



attains F ; s 2 32. The anatomy of the Holothuria tubutoia. 



