498 HERRICK E. WILSON 



way (see p. 502), until it develops into a round, cubiform plate, the 

 anal plate (Fig. 2, No. 3). 1 The radials, with the anal plate between, 

 now form nearly a complete circle, resting on the basals and sepa- 

 rating them completely from the orals. 2 Although greatly enlarged, 

 the radials are still subquadrangular in outline, the proximal angle 

 occupying the enlarged portion of the interbasal suture, and the 

 distal angle, now trunkated, supporting the narrow costals. 3 Con- 

 siderable space still exists between the adjacent radials, except 

 where they are in apposition with the anal plate (Fig. 2, No. 4), 

 and these spaces are rilled only with sarcodic substance. 4 The anal 

 plate from proximal growth now comes into apposition with the 

 posterior basal, and the two are mutually trunkated. Upon fur- 

 ther development the radials meet, and their margins assume the 

 finished appearance previously noted in the proximal portion of 

 the basal plates. The posterior radials, especially the right, how- 

 ever, show marked asymmetry, owing to the non-development of 

 the sides adjacent to the anal plate (Fig. 2, No. 5). Growth in 

 the radials does not cease upon their meeting, and as the basals 

 do not now further enlarge, the radials and the anal are forced by 

 contact with each other to extend themselves in an oblique direc- 

 tion, thus enlarging their distal perimeter, and increasing the 

 diameter of the tegmen. 



The anal plate by this time has reached its full development 

 and, being more firmly attached to the visceral mass than to the 

 adjacent radials, 5 is gradually lifted out of the cup by the extension 

 of the anal tube. The space left by the withdrawal of the anal is 

 gradually rilled by lateral growth from the adjacent radials, which, 

 however, do not immediately come into contact. Before the anal 

 is completely withdrawn from the radial cycle, however, the pos- 

 terior radials meet below it, and, as withdrawal continues, cor- 

 responding and continuous enlargement of the radials tills the 

 re-entrant angle, and gives to the plates their bilaterally sym- 

 metrical outline (Fig. 2. No. 6). 



About this time a very remarkable change takes place in the 

 tegmen. The oral cycle, like the basal one, does not partake of 



1 Ref. 35, p. 529 (anal). ; Ref. 11, p. 729. 



- Ref. 15, p. 314 (radianal). * Ref. 11, p. 729. - ; Ref. 11, p. 73:. 



