36 EMBEYOLOGY OF THE STARFISH. 



develop across the surface of an elliptical tube, the five folds naturally 

 form a twisted spiral, with a pentagonal outline, each side of this spiral 

 forming the first nucleus of the five ambulacral tubes. I speak con- 

 stantly of pentagonal spirals, pentagonal ambulacral system, and pentago- 

 nal abactinal system. In using these terms, I do not mean a pentagon 

 with five equal sides, the adjacent sides making equal angles with one 

 another and surrounding a closed surface, but simply that we have five 

 sides limiting an open space, the two extremities of this five-sided figure 

 being separated by the whole vertical diameter of the water-tubes. One 

 extremity of the ambulacral five-sided open figure is placed at the water- 

 pore {b, PL v. Fig. 2), the other at the opposite side of the water-tube 

 on the surface of which the ambulacral system is developed. The two ex- 

 tremities of the abactinal open five-sided figure are placed, one above the 

 water-pore {b, PL V. Figs. 8, 9, 13, ?•'['), the other on the opposite side of the 

 water-tube, which develops the abactinal surface on one side of the anus 

 (a, PL V. Fig. 14, ?•'•'). A glance at the figures of the Brachiolaria from the 

 dorsal or ventral side (PL IV. Fig.s. 1, 2 ; PL VII. Fig. 8 ; PL V. Fiffs. 8, 

 9, 13, 14) shows that the two surfaces, upon which the actinal and abac- 

 tinal areas are developed, do not correspond to one another, or fit into 

 each other as in the full-grown Starfish. That is, if the ambulacral system 

 were projected upon the abactinal system, in order to bring these two 

 surfaces into the same relation which they hold in the adult Asteracan- 

 thion, we should find the ambulacral system projecting beyond the out- 

 line of the abactinal system, and placed nearer the mouth of the Brach- 

 iolaria, while a portion of the abactinal system — that which is placed at 

 the anal extremity of the larva — would, in the same way, project be- 

 yond the outline of the ambulacral system. 



The sides of this twisted pentagonal spiral are somewhat concave, and 

 the apex of the angles of adjacent sides are rounded. It is in conse- 

 quence of the changes taking place at the apex of the sides of this irregu- 

 lar ambulacral pentagon that we have the simple apex transformed, by 

 its gradual extension beyond the general outline of the open pentagon, 

 into the five-folded loops (PL V. Figs. 10, 12), each of which corresponds 

 to an ambulacral tube and its accompanying suckers in an adult Star- 

 fish. 



The ambulacral pentagon with concave sides and rounded angles, seen 

 in profile (PL V. Figs. 2, 5 ; PL III. Figs. 7, 9, t), changes its shape 



