30 EMBRYOLOGY OF THE STARFISH. Part I. 



constantly of pentagonal spirals, pentagonal ambulacral system, and pentagonal abac- 

 tinal 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, PI. 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 extremities of the abactinal open five-sided figure are placed, one above 

 the water-pore (b, PL V. Figs. 8, 9, 13, r"), the other on the opposite side of the 

 water-tube, which develops the abactinal surface on one side of the anus (a, PI. V. 

 Fig. 14, r'l'). A glance at the figures of the Brachiolaria from the dorsal or ventral 

 side (PI. IV. Figs. 1, 2; PL VII. Fig. 8; PL V. Figs. 8, 9, 13, 14) shows that 

 the two surfaces, upon which the actinal and abactinal 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 Astera- 

 canthion, we should find the ambulacral system projecting beyond the outline of 

 the abactinal system, and placed nearer the mouth of the Brachiolaria, while a portion 

 of the abactinal system — that which is placed at the anal extremity of the larva 

 — would, in the same way, project beyond 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 consequence of the changes 

 taking place at the apex of the sides of this irregular ambulacral pentagon that 

 we have the simple apex gradually 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 Starfish. 



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 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 ruffling of their folds (PL III. Fig. 11), and 

 each apex of the pentagon has the appearance of a small loop projecting beyond 

 the curved sides; the loops grow larger 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; PL IV. Fig. 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 (PL V. Figs. 4, 



