INTERNAL STRUCTURE OF THE SEA-URCHIN. 97 



radulse, and articulating with them centrally, while their 

 peripheral ends are free. 



(iv.) The ten epiphyses, articulating with the ends of the 

 radulse. 



(v.) The free inner ends of the five teeth. 



7. Make a drawing showing all these points. 



8. Remove one of the alveoli, and in a side view 

 notice : 



(i.). The flat surfaces (Fig. 42, b] by which adjacent 

 alveoli face each other. 



(ii.) The parallel horizontal ridges for the 

 attachment of the concentrator muscles. 



(iii.) The groove or joint along the upper 

 edge, for articulation with the radulae. 



FIG. 42. Side view of an alveolus. (Drawn from na- 

 ture by H. J. Rice. ) 



6. Flat surface of alveolus, c. Outer surface, d. Tooth. 



(iv.) The open space between the central edges of the 

 halves of the alveolus. 



(v.) The long tooth (Fig. 42, d) set into the socket 

 formed by the alveolus. Take out the tooth and notice : 



(a.) The exposed pointed cutting edge. 



(5.) The ridge or keel along the inner surface. 



(c.) The long, imbedded, growing portion of the tooth. 



c. The muscles of the lantern may now be examined in 

 an alcoholic specimen. Notice : 



1. The five transverse muscles (Fig. 40, c) which con- 

 nect the five radii with each other on the inner surface of 

 the lantern. 



2. A pair of tendons (Fig. 40, d) running outwards 

 and downwards from the outer end of each radius to the 

 inter-ambulacral areas of the inside of the shell. 



