OR OSSEOUS SYSTEM. 



rangement of the plates is best seen, is represented in Fig. 

 8, 1 , where a, a, represents the smooth inner surface of the 

 large tuberculated plates, and FIG. 8. 



b, by the inner surface of the a ^j_y-^-^^^-^-~~^^^M 

 perforated ambulacral plates. 

 There are ten vertical co- 

 lumns of tubercular and ten 

 of ambulacral plates which 

 vary in the number of pieces 

 they contain, according to the 

 <age of the animal. The 

 larger plates are the tuber- 

 cular a, a, which have a pen- 

 tagonal form, and are length- 

 ened transversely. There are 

 about thirty-two of these tubercular plates in the adult ani- 

 mal, in each of the vertical columns, making three hundred 

 and twenty plates of this form in the ten columns. The 

 columns of tubercular plates meet each other by obtuse, 

 salient and re-entrant angles, as seen at d in Fig. 8, 1, form- 

 ing thus a regular zig-zag line by the contiguous columns of 

 these plates, and the shell is much strengthened by the al- 

 ternate manner in which the sutures are arranged. The am- 

 bulacral plates form also five pairs of columns (Fig. 8, 1, b,b,) 

 each column containing about eighty pieces. The tubercular 

 columns are united to the ambulacral by minutely serrated 

 edges, and the ambulacral columns are united to each other by 

 the same form of zig-zag suture as that between the tubercular 

 plates. The ambulacral plates however (Fig. 8, 1, b,b,) appear 

 to have their perforated portions detached by small sutures 

 which traverse all the foramina, (Fig. 8, 1, c, c.) There are thus 

 ten columns of these minute perforated pieces, each column 

 having one hundred and sixty pieces. The contiguous portions 

 of the ambulacral pieces, (Fig. 8, 1, b 9 by) are covered externaUy 

 with tubercles like the larger tuberculated pieces. (Fig. 8, 1, a,a.) 

 When we examine the fractured edges of these shells, we 

 observe the colouring matter of the surface to pass under the 

 superficial tubercles, which appear not to be continuous por- 

 tions of the plates to which they adhere by a broad spreading 

 circular base. The tubercles are few on the small terminal 

 plates, but numerous on the large pieces in the middle of 



c 2 



