28 EMBRYOLOGY OF THE STARFISH. Part I. 



such a projection is that of a warped spiral, so that, in reality, when passing (in 

 PL V. Mg. 10) from r'{', along the edge of the disk, to tf, r' 3 ", rf , r'i' , we # do not 

 move in a plane, but are constantly winding, somewhat as when ascending a spiral 

 staircase ; this is seen in PI. V. Fig. 9, when passing from rf , the arm placed 

 nearest the anus, along the edge of the abactinal area, to r{", the arm next to 

 the water-pore (b). It is the same for the actinal arc, which forms a spiral iden- 

 tical to that of the abactinal area, only bent in the opposite direction. 



The actinal and abactinal regions are, in reality, two warped spiral surfaces, 

 making an angle with one another, separated by the whole width of the stomach. 

 This is best seen in a view from the dorsal or oral side (PL VII. Fig. 8), when 

 the folds are distinctly visible one above the other, but so arranged as to be all 

 seen at the same time (PL V. Figs. 4, 6, 7, 8; PL III. Figs. 8, 10, 11). Three of 

 the folds are near the edge, while the other two are placed close to the digestive 

 cavity on the ventral side. This spiral, seen from the dorsal or from the ventral 

 side, has all the appearance of the foot of a bivalve (t, PL V. Figs. 4, 6, 8). The 

 spiral position of the five rods indicating the position of the future rays of the 

 Starfish {r'{'-r'l') is also apparent from the same point of view. Two of the rods 

 are placed on the dorsal side of the larvse, running somewhat obliquely (/{', r'i'\ 

 the three others (r 3 ", /{', r'l') turning away still more from the median line ; the 

 last {//) placed very near the edge, on the ventral side, close to the base of the 

 median arms (PL V. Figs. 3, 5, 6, 8, 9, r"-r'l'); the nearest distance between these 

 two spiral surfaces being fully as great as the width of the water-tube: in fact, it 

 seems as if the rudimentary tentacles and the dorsal system had as yet no connec- 

 tion whatever with one another (PL V. Figs. 6, 8). 



It is very important that this oblique position of the actinal and abactinal areas, 

 as well as their great distance apart, should be distinctly kept in mind; as it will 

 explain many of the errors committed by previous writers on this subject, and 

 greatly assist us in correctly understanding many points in the anatomy of Echino- 

 derms hitherto unexplained. 



From what has been shown thus far, it is self-evident that the water-tubes, the 

 problematic bodies, as Miiller has called them in their early condition, are the sur- 

 faces from which the future Starfishes are developed, and not the surface of the 

 stomach. The spiral of tentacles is developed by folds placed on one side of the 

 stomach (PL III. Figs. 6, 8, 10, 11), on one of the water-tubes (w'), that with the 

 water-pore (b) ; while round the other water-tube (w), placed on the other side of the 

 stomach, is formed the spiral surface of the abactinal system. The stomach has 

 remained as it was before, and has in no way contributed to the formation of the 

 young Starfish. A glance at any figure of the larvse, either in profile or from 

 above or from below, will show that no change has taken place in the shape of the 



