32 EMBRYOLOGY OF THE STARFISH. Part I. 



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"). 

 The small granular cells, rilling the larger meshes of the net-work, increase in number, 

 rendering the whole abactinal system somewhat opaque ; when 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 follows 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 pentagon (t), and only the lower oral 

 edge of the abactinal system; the net-work of limestone meshes being quite indis- 

 tinct, as seen through the thickness of the abactinal surface (PI. III. Fig. 7 ; PI. IV. 

 Fig. 4 ; PI. V. Figs. 10, 12), while, in the other case, a full view of the abactinal 

 pentagon (PL III. Fig. 10 ; PL IV. Fig. 4 ; PL V. Figs. 5, 11) is obtained, and the 

 arrangement of the different rods forming the plates of the limestone net-work is 

 distinctly seen. A view of the larva from the dorsal side (PL III. Fig. 11 ; PL VII. 

 Fig. 8 ; PL V. Figs. 8, 9, 13) shows the abactinal system extending in such a 

 way as to surround the stomach entirely on one side, while the tentacular pentagon 

 covers it on the opposite side. This attitude gives us the position of the lobes 

 ( r '\- r 'l'\ the future rays of the Starfish, next to the water-pore {/{', r'i'\ while a 

 view from the oral side (PL V. Figs. 4, 14) indicates the trend of the lobes on the 

 opposite extremity of the spiral of the abactinal system {r'l', r b "). 



Formation of the Bags of the future Starfish. — The plates of the abactinal system 

 early reach a condition when the changes they undergo are merely quantitative, and 

 the only modifications affecting the appearance of the Starfish take place on the 

 edge of the disk. A depression is formed in the middle of the convexity of the 

 lobes of the abactinal area ; this is soon followed by two other depressions in 

 the middle of the small arcs thus formed, dividing each lobe of the pentagon into 

 four smaller lobes; at the same time the indentations between the original sides of 

 the pentagon have grown much deeper, separating these five lobes in a very marked 

 manner. We can now no longer mistake the true character of the lobes; they are 

 the five rays of the Starfish, but as the actinal and abactinal regions are not yet 

 fitted together, as we find them in the adult (PL V. Figs. 10, 11, r'i'-r'l' ; PL IV. 

 Fig. 4), they represent only the dorsal sides of the rays. A glance at Fig. 9 of the 

 same Plate (PL V.), seen from the dorsal side, will show how far the suckers if) are 

 removed from the abactinal portion of the arm which is to protect them. The 



