THE SEA URCHIN. 63 



tree. Sections of them form exquisitely beautiful microscopic 

 objects. 



Holding up the empt}^ " test " to the light, we can plainly 

 see the numerous minute holes of which we have spoken, all 

 arranged in a symmetrical pattern. 



Through these, in the living animal, are protruded those 

 wonderful organs, the sucker feet or ambulacral feet, which 

 enable it to attach itself firmly to any substance, and are also 

 its chief organs of locomotion ; and further even to right itself 

 if placed, so to speak, on its back. Those near the mouth are 

 also said to perform the additional function of seizing prey 

 and dragging it to the mouth of the Echinus. No less than 

 3,840 of these ambulacral suckers have been counted in 

 a single individual. And now for their mode of working. 

 AVithin the "test" are certain canals passing along like 

 meridian lines immediately below the rows of pores. At 

 the base of each pair of pores is a small bag filled with salt 

 water. At the will of the animal this is forced into the 

 suckers, and these can thus be protruded far beyond the 

 spines. When the muscles of the bag relax, the pressure is 

 removed, the water flows back from the tube foot, and it 

 contracts. The end of each sucker is stiengthened by a 

 calcareous disc or rosette of the most delicate network — a 

 " thing of beauty " under the microscope. 



Now, a word or two about these curious objects, the 

 Pedicellarias, which look like parasitic polyps (as indeed they 

 were at one time thought to be) attached to the spines. 

 These minute organs consist of a sort of forceps, having three 

 blades or jaws, the inner edge of which is provided with 

 irregularly shaped teeth. These calcareous nippers are 

 attached by a long flexible stalk to the Echinus, and they 

 are seen constantly waving about, and the jaws opening and 

 closing. Their power of " holding on " is amazing, and thus 

 they are eminently fitted for seizing hold of seaweed, &c, till 

 the sucker feet have time to attach themselves. Indeed 

 Professor Romanes, who has experimented largely with Echini, 

 considers this the chief function of the Pedicellarise. Even 

 when separated from the living animal they continue their 

 " grasping " habit for some time. 



Turning now to the mouth of the Sea Urchin, we find in 

 the true Echini that the mouth is in the centre of the lower 

 surface, and is armed with teeth. In the heart urchins the 

 mouth is eccentric and edentulous. The teeth resemble those 

 of the Rodents in form, and continue to grow throughout the 

 animal's life. They are five in number, and very hard. The 



