34 EMBEYOLOGY OF THE STAEFISH. 



oral side (PI. YII. Fig. 8), when the folds are distinctly visible one above 

 the other, but so arranged as to be all seen at the same time (PI. V. 

 Figs. 4, 6, 7, 8; PI. HI. 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, PI. V. Figs. 4, 6, 8). 

 The spii'al position of the five rods indicating the position of the future 

 rays of the Starfish {r'i' -r"^) is also apparent from the same point of 

 view. Two of the rods are placed on the dorsal side of the larvae, run- 

 ning somewhat obliquely (r'i', r"^), the three others {r"z, r"l, r"^) turning 

 away still more from the median line ; the last (r's') 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'-l-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 tlie dorsal system 

 had as yet no connection whatever with one another (PL V. Figs. 6, 8). 



It is very important that this oblique position of the actinal and abac- 

 tinal areas, as Avell as their great distance apart, should be distinctly 

 kept in mind ; as it will explain many of the errors committed by pre- 

 vious writers on this subject, and greatly assist us in correctly understand- 

 ing many points in the anatomy of Echinoderms hitherto unexplained. 



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

 tubes, the problematic bodies, as MUller has called them in their early 

 condition, are the surfaces 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 {to'), 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 re- 

 mained as it was before, and has in no way contributed to the foi'mation 

 of the young Starfish. A glance at any figure of the larvfB, either in pro- 

 file or from above or from below, will show that no change has taken 

 place in the shape of the stomach, or any part of the alimentary canal, 

 as Miiller believed (PL V. Figs. 1, 8; PL III. Figs. 1-11), but that a 

 kind of cap has been formed lound it by the water-tubes. Owing, how- 

 ever, to the accumulation of very fine granules of limestone, the anal 

 extremity has by this time lost its transparency ; this would be easily 



