354 ECHINODERMATA [CH. 



skeleton it is split into two parallel tubes which reunite below 

 (B, Fig. 164) : so that on the dried shell we see on the ambulacral 

 plate several pairs of pores, each pair corresponding to a single 

 tube-foot. As the cells lining the inside of the latter are ciliated, 

 the splitting of the tube is apparently for the purpose of facilitating 

 the separation of the upward and downward currents of water. 



The peristome has ten branched pouches, situated one pair in 

 each interradius and projecting outwards. These are the gills, 

 but it is unreasonable to suppose that all the breathing is done by 

 them. They communicate not with the general body-cavity, but 

 with a part of it, called the lantern coelom, shut off from the 

 rest by a septum stretched between the teeth and jaws. Embedded 

 in the upper wall of this are certain rods called compasses, which 

 are connected with each other and with the auriculae by muscles, 

 and by means of these the upper wall of the lantern coelom can be 

 raised or depressed and the pressure inside altered. When these 

 rods are depressed water is driven out into the gills and there 

 absorbs oxygen. When they are raised the water is sucked back 

 into the lantern coelom and the oxygen passes through the thin 

 wall of the latter into the general coelom. A very curious use of 

 the jaws and teeth has recently been demonstrated by Dr Gemmill. 

 When the urchin gets into shallow water and is partly left dry by 

 the tide, then a peculiar rhythmic action is set up in the teeth. 

 The whole lantern is swung forward by the pull of the muscles 

 attaching it to the auriculae. Then the teeth are protruded by the 

 action of the protractor muscles and firmly inserted in the ground, 

 and using the protruded teeth as a fulcrum or as the point of a 

 vaulting pole the body of the urchin tumbles forward. The teeth 

 are then retracted, the lantern is again swung forward and the 

 action is repeated, and in this way the urchin progresses by a series 

 of short leaps. When once this action has been set up it continues 

 for some time after the urchin has been covered with water. 



The organs of sex are alike in external appearance in both sexes 

 (Fig. 165). They have the form of five great bunches of tubes 

 hanging down into the body-cavity and opening by five small holes 

 placed in plates called the genital plates, forming the summit of 

 the interambulacral series on the corona and situated just outside the 

 periproct (Figs. 160 and 167). In the young Sea-urchin there is a 

 genital rachis connecting them together, and throughout life there 

 is a genital stolon alongside the stone-canal. The genital stolon 

 is relatively much larger in Echinoidea than in Asteroidea, and 



