38 EMBRYOLOGY OF THE STARFISH. 



granular deposit of limestone ; and the edge of the surface, connecting 

 the two extremities of the abactinal pentagon, can readily be seen in 

 profile (PI. V. Fici. 5). The five large rods placed in the middle of the 

 sides of the spiral abactinal pentagon, and the five small ones placed in 

 the angles of this same pentagon, are the first trace of the plates com- 

 posing the abactinal surface of the young Starfish. 



The water-pore {h, PI. III. Fig. 10 ; b, PI. V. Fir/s. 7, 8) remains open, 

 the only change being an accumulation of limestone matter round the 

 opening, forming a sort of solid tube to protect it. This water-pore, as 

 we shall see hereafter, eventually becomes the madreporic body ; and 

 the canal formed by the deposition of limestone is the stone canal of the 

 full-grown Starfishes. 



Abactinal System. — The double line on the edge of the abactinal pen- 

 tagon (PI. V. Fig. 2) is formed by the thickness of the surface of the 

 abactinal system. This double line, at first only slightly vnidulating, be- 

 comes gradually more indented (PI. V. Figs. 3, 5) ; at the same time, 

 additional rods arise round the primary ones with such rapidity that we 

 soon find a complicated network of limestone rods, forming ten clusters 

 (PI. V. Figs. 8, 9, 13, r, ?•"), five large (?•') and five smaller ones [r") round 

 the original rods. This network is produced by the addition of a Y- 

 shaped rod, at each extremity of a simple primary rod ; presently, eight 

 Y-rods arise upon the shanks of the first set of Y-rods, followed by a 

 third set upon the shanks of the second set, and so on ; in this manner 

 are formed the closed polygons composing the clusters of the patches of 

 limestone deposit (PI. V. Fig. 9, r, r"). The small granular cells, filling 

 the larger meshes of the network, increase in number, rendering the 

 whole abactinal system somewhat opaque ; Avhen the larva is seen in 

 profile from the abactinal side, the outline of the stomach (PI. V. Fig. 

 5) can be traced exactly as it was before the Starfish had begun to form ; 

 and outside of it, the edge of the future back is distinctly visible (PI. V. 

 Fig. 5). 



As the two water-tubes are placed on opposite sides of the larva, it fol- 

 lows that when seen in profile (PL V. Figs. 11, 12), from the left or from 

 the right, it presents, in the one case, a full view of the tentacular pen- 

 tagon (t), and only the lower oral edge of the abactinal system, the net- 

 work of limestone meshes being quite indistinct, as seen through the 

 thickness of the abactinal surface (PI. 111. Fig. 7 ; PI. IV. Fig. 4 ; PI. V. 



