32 EMBRYOLOGY OF THE STAEFISH. 



Up to the stage of the larva represented on PI. III. Figs. 6, 7, the out- 

 line of the kji water-title (left when seen from above in its natural attitude), 

 in a profile view, is that of a flattened cylinder (PI. V. Fig. 1, w'), Avith 

 the end slightly bent towards the anal opening. Near the point where 

 the upper line of the water-tube bends downwards, a marked indenta- 

 tion is formed, having in the centre a slight projection. There appear, 

 soon after starting from the anal edge of this depression, five very 

 faintly defined folds, the first trace of the future ambulacral sj^stem, ex- 

 tending obliquely across the water-tube (?y') (PI. V. Fig. 2, t ; PI. III. 

 Fig. 8). If we examine the other side of the anal extremity, we find 

 deposited opposite the angles of these folds (PI. V. Fig. 2, /), five rods 

 of limestone ; the anal jiart of the larva having at the same time lost 

 its former transparency, and assumed a dull yellow color. These two parts 

 ai'e the first traces of the future Starfish. The limestone rods, and the 

 whole of the granular surface covering the rigU water4ube, with the dorsal 

 pore, forms eventually the abactinal area of the adult Starfish ; while the 

 folds, running obliquely across the left irater-tiibe, are the first rudiments 

 of the future rows of suckers extending along the lower side of the fu- 

 ture rays ; the rods are placed exactly opposite what will hereafter be 

 the extremity of the rays. 



It is apparent, from the above description, that the abactinal area (the 

 rods), and the suckers (the folds across the water-tube), are not situated 

 in one plane, or even in parallel planes. The arc containing the rods 

 and the arc passing through the folds make an acute, nearly a right 

 angle, as is better understood by referring to older stages. It will also 

 be seen, by a glance at the drawings (PI. V. FiffS. 1, 2, 3, 5; PI. III. 

 Figs. 7 - 10, t), that the folds denoting the place where the suckers will 

 make their appearance, and the rods (/, r") marking the position of the 

 future rays, are neither of them closed curves, but are always open, forming a 

 sort of twisted crescent-shaped arc. When describing the young Starfish 

 immediately after it has resorbed the larva, and is ready to crawl about 

 by means of its suckers, I shall show how these curves become closed ; 

 and also point out the changes these parts undergo to form diverging 

 rays, as well as the manner in which the warped surfaces developing the 

 actinal and abactinal regions are brought into parallel planes. 



Relative Position of the Actinal and Abactinal Areas. — The folds of the water- 

 tube {w'), which forms the actinal area, are not contained in one plane, 



