WATER VASCULAR SYSTEM OF ECHINUS 67 



result. The animals were apparently not healthy ; the whole 

 of the interior stained, but there was no sign of any colour 

 being excreted anywhere. 



These experiments seemed to support Hartog's view, but 

 were by no means conclusive. 



In 1915 I carried out other experiments, and have con- 

 tinued them either at Easter time or during the summer, 

 every year since then, at Port Erin. 



It seemed doubtful whether evidence obtained from 

 dissected madreporites and. stone canals could be relied upon ; 

 therefore in the following experiments dissection has been 

 avoided as much as possible. 



Experiment 1. Stone canals were examined under a 

 binocular microscope without being removed from the animal. 

 This is easily done by cutting a window on each side of the 

 animal and examining it in a glass vessel filled with sea water. 

 The currents seen agreed with our previous observations ; 

 there is a peripheral current towards the oral surface, apparently 

 caused by cilia lining the stone canal, and a central current 

 towards the madreporite. It is impossible to decide from 

 observation which of these is the main current, but the fact 

 that both are present perhaps accounts for the directly opposite 

 reports of Hartog and Ludwig. Observations made with a 

 binocular microscope leave no doubt on this point. 



Experiment 2. living specimens of Echinus were 

 examined in sea water with charcoal or carmine in suspension. 

 No particles were ever seen to settle on the madreporite, 

 although they settled freely elsewhere. A large number of 

 specimens have been examined in this way, and always give 

 the same result. As one observes the phenomenon the impres- 

 sion strengthens that the action is due to cilia on the surface, 

 and not to a steady outward current, in spite of our previous 

 experiment with a well-scraped madreporite. 



Experiment 3. To test for surface ciliary action the 



