FORMATION OF THE ABACTIXAL SYSTEM. 37 



rapidly ; the convex cavity becomes greater, the apex of each angle of 

 the pentagon more prominent and less pointed, a double line is formed 

 by the ruflling of their folds (PI. III. Firj. 11), and each apex of the pen- 

 tagon has the appearance of a small loop projecting beyond the curved 

 sides ; the loops grow la.'ger and larger, until they have reached a size 

 somewhat less than one third of the diameter of the water-tube, when 

 they stand out freely from the pentagon, and seem to form no part of 

 the water-tube (PL V. Figs. 10, 11, 12, t ; Pi. IV. FUj. 4). When seen 

 either from above or from below, the folds appear as small flaps on the 

 broad side of the foot-like appendage projecting from the surface of the 

 stomach, formed by the folding of the water-tube (PI. V. Figs. 4, 6, 8, 9, 

 13, 14, ^; PI. III. Fiys. 10, 11 ; PI. IV. Figs. 1, 2; PI. VII. Fig. 8). These 

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

 the main water-tube [w] ; 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 suckers (PI. V. Figs. 10, 11, 

 12, t t t t t). The ambulacral pentagon remains in this state until the 

 Starfish has resorbed the many appendages of the larva. 



Formation of the Ahactinal System. — Let us now follow the corresponding 

 changes of the abactinal system, accompanying the modifications, just de- 

 scribed, of the ambulacral pentagon. On examining the anal extremity, 

 at the time when the larva has reached the state represented on PI. 

 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 imiform 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 corre- 

 sponding to the apex of the pentagon of suckers (PI. V. Fig. 5, r'i-r'i', PI. 

 III. Fig. 10). The rods, simple at first (/, PI. 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 con- 

 cavities of the lobes of the actinal system, a second set of small rods (/•", 

 PI. V. Fig. 5), of a similar character to the large ones, have developed. 

 The whole of the abactinal svstem has become coated with a very fine 



