526 



ECHINODERMATA ECHINOIDEA 



ectoderm and project into the lower end of the oesophagus. 

 Each tooth is firmly fixed by a pair of ossicles inclined towards 

 one another like the limbs of a V and meeting below. Each ossicle 

 is called an " alveolus," and taken together they form a " jaw. 

 Their upper ends are connected by a pair of ossicles called 

 " epiphyses " (13). These two epiphyses meet in an arch above. 

 The jaws and their contained teeth are situated interradially. 

 Intervening between successive alveoli are radial pieces called 

 " rotulae," which extend directly inwards towards the oesophagus. 

 Above the rotulae are pieces termed " radii " or " compasses " (2), 

 which are not firmly attached to the other pieces but lie loosely 

 in the flexible roof of the lantern-coelom. 



The uses of the various components of this structure can be 

 made out from an inspection of the muscles which connect them 

 together. 



Overarching each radial perihaemal canal where it leaves the 

 lantern is a bridge of calcareous matter called the "auricula" 

 (Fig. 234, 8). This arises as two rods which meet each other 

 in a pent-house over the canal. It is the only part of the 

 skeleton which can be compared to the ambulacral ossicles of the 

 Asteroidea, and like them it serves as the point of insertion for 

 important muscles. Thus we find (1) protractor (Fig. 234, 7) 

 muscles which arise from the upper ends of the alveoli and are 

 inserted in the auricula ; when these contract they tend to push 

 the whole " lantern " outwards so as to expose the tips of the 

 teeth. (2) The retractor muscles (Fig. 234, 6) extend from the 

 auriculae to the lower ends of the jaws and restore the lantern 

 when it has been extruded to its original position. (3) The 

 comminator muscles connect adjacent jaws with one another : 

 these on contraction approximate the pair of jaws into which 

 they are inserted, and it will easily be seen that by the successive 

 contraction of the five comminator muscles a rotating movement 

 of the teeth would be produced which would cause them to exert 

 an action something like that of an auger ; by their simultaneous 

 contraction the teeth are brought to a point. (4) The internal 

 and external rotula muscles : these are small muscles which 

 connect the outer side of the epiphysis with the rotula. There 

 are two facets on the epiphysis, which permit it to rock to 

 and fro on the rotula under the action of these muscles. This 

 rocking action must greatly increase the cutting power of the 



