Chap. II. ABACTINAL SYSTEM. 31 



6, 8, 9, 13, 14, t; PL III. Figs. 10, 11; PL IV. Figs. 1, 2; PL VII. Fig. 8). These 

 small folds are, in reality, nothing but open bags communicating with the main 

 water-tube (iv) ; small pouches leading from it. The outer and inner fold of each 

 loop do not remain concentric, and we can soon trace, in the inner fold, changes 

 similar to the first folding of the water-tube. The rounded end of the inner fold 

 becomes triangular; this is the first indication of the formation of the separate suck- 

 ers (PL V. Figs. 10, 11, 12, tttt t). The ambulacral pentagon remains in this state 

 until the Starfish has resorbed the many appendages of the larva. 



Formation of the abactinal System. — Let us now follow the corresponding changes 

 of the abactinal system, accompanying the modifications, just described, of the am- 

 bulacral pentagon. On examining the anal extremity, at the time when the larva 

 has reached the state represented on PL III. Fig. 10, we are at once struck with 

 the fact that the outline of the abactinal system has undergone analogous changes 

 to those of the actinal pentagon. Instead of remaining a uniform spiral, the two 

 ends of which are separated by the whole height of the water-tube, while the two 

 areas are divided by the combined width of the stomach and the two water-tubes, 

 it has a slightly-lobed pentagonal outline, the convexities corresponding to the apex 

 of the pentagon of suckers (PL V. Fig. 5, r{'-r'i' ; PL III. Fig. 10). The rods, 

 simple at first (r, PL V. Fig. 2), have increased in size; small Y-shaped appendages 

 have developed at their' extremities. We also see that in the intermediate spaces, 

 corresponding to the concavities of the lobes of the actinal system, a second set of 

 small rods (/', PL V. Fig. 5), of a similar character to the large ones, have developed. 

 The whole of the abactinal system has become coated with a very fine granular de- 

 posit of limestone ; and the edge of the surface, connecting the two extremities of 

 the abactinal pentagon, can readily be seen in profile (PL V. Fig. 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 composing the abactinal surface of the young Starfish. 



The water-pore (b, PL III. Fig. 10 ; b, PL V. Figs. 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 pentagon (PL V. 

 Fig. 2) is formed by the thickness of the surface of the abactinal system. This 

 double line, at first only slightly undulating, becomes gradually more indented (PL 

 V. Figs. 3, 5) ; at the same time, additional rods arise round the primary ones with 

 such rapidity that we soon find a complicated net-work of limestone rods, forming 

 ten clusters (PL V. Figs. 8, 9, 13, /, r"), five large (/) and five smaller ones (r") 



