xvni ECHINUS ARISTOTLE'S LANTERN 527 



tooth. These muscles are controlled by the nerve-ring and the 

 incipient portions of the radial nerves, which, as we have seen, 

 have an inner layer of nerve-cells. If the nerve-ring be gently 

 stimulated on one side the upper end of the lantern bends away 

 from the spot, causing the lower end, i.e. the teeth, to move 

 towards it ; but a stronger stimulation produces the opposite 

 effect, just as is the case with spines. . But besides these 

 masticatory muscles there are others which have nothing to do 

 with moving the teeth. These muscles are attached to the rods 

 called radii or compasses (Fig. 234, 2),^ which lie in the upper wall 

 of the lantern-coelom, and may be termed the compass muscles. 

 There are two sets : — (1) The elevator muscles (Fig. 234, 3), which 

 connect .the inner ends of the compasses with one another. 

 When these contract, the radii tend to bend upwards at the inner 

 ends and thus raise the roof of the coelom. (2) The depressor 

 muscles (Fig. 234, 4), which run downwards from the forked outer 

 ends of the compasses to the auriculae. Uexklill^ has shown 

 that the function of these muscles and of the rods to which they 

 are attached is respiratory. These muscles are also controlled by 

 the nerve-ring. If this be stimulated by passing a pin-head 

 into the oesophagus, the roof of the lantern cavity is raised by 

 the contraction of the elevator muscles. This is followed by 

 contraction of the depressor muscles lowering it ; the same result 

 may be brought about by placing the animal in water with excess 

 of carbonic acid. The ten branched gills described on p. 5 14 are 

 outgrowths of the lantern-coelom. When the roof of this cavity 

 is depressed the fluid contents are driven out into the gills, 

 which are thus expanded and then absorb oxygen from the 

 surrounding sea water. When, on the other hand, the roof is 

 raised the aerated water is sucked back into the lantern cavity, 

 and the oxygen passes easily through the thin walls of the lantern 

 into the fluid filling the main coelomic cavity. There are thus 

 two independent respiratory mechanisms in the Sea-urchin, the 

 one being the compass muscles, the other the cilia lining the 

 interior of the tube-feet. 



The function of excretion is performed, as in Asteroidea, by 



1 We prefer the term "compasses," to avoid confusion with the other meanings 

 of the word "radius." 



2 "Ueber die Function der Polisclien Blasen am Kauapparat der regularen 

 Seeigel," MiOh. Zool. Stat. Neapel, xii. 1897, p. 464. 



