Plate IX. 



Fig. 107. Young sea-urchin, sixty days old. Dorsal view. Length of the body including the spines: 

 fröna 0,9 mm. to 1 mm. a: anterior extremity; p: posterior extremity; o: water-pore. 



» 108. The peristomal region of a young sea-urchin somewhat more advanced th.-« that in fig. 

 105, showing the deutary apparatus inside the buccal membrane, which still remains 

 entire and unpierced. Seen from the under surface. Length of the buccal space: 

 0,18 mm.: breadth of the same: 0,i6 mm.; diameter of the dentary apparatus: 0,i4 mm. 

 a; frontal ambulacrum; p: posterior interambulacrum; i: interradial plates; r: radial 

 plates: 3: spherids in their receptacles; t: teeth; z: radial pieces of the lantern in 

 its early developmental stage. 



» 109. The peristomal region of a sea-urchin of the same size and age as that in fig. 107. 

 Diameter of the buccal space, 0,24 mm. The ends of the teeth protrude through the 

 opened pentagonal mouth. 1: threads; m: muscles. The remaining letters as in the 

 preceding figure. 



» 110 a — e. Five developmental stages of the sphseridia. a — d: side-view; e: seen from above. 

 Length from 0,oi to 0,oi2 mm. 



» 111. Four developmental stages of a tooth, a: the first trace of a tooth seen from the outer 

 convex surface and measuring O.oos mm. in length and breadth; b: a very young tooth 

 seen from the inner concave surface where a very minute deposit appears indicating 

 the future inner laminaä; c: a young tooth, 0,o24 mm. long, showing two small laminse 

 inside the first formed one; d: a tooth somewhat more advanced. 



» 112. Tooth of a sea-urchin somewhat younger than that in fig. 107. Seen from the inner side 

 Length: 0,o9 mm. Breadth: 0,o24 mm. 



