EELATIVE POSITION OF THE AEEAS. 33 



but are placed upon a spiral; the same is the case with the five lime- 

 stone rods situated on the surface of the other water-tube (w), which 

 forms the abactinal region. When we look at the Brachiolaria from the 

 side, that is, when facing the groove which separates the ventral from 

 the dorsal side, as in PI. IV. Fig. 4, or in the corresponding profiles, from 

 the side of the right and left water-tubes of PI. V. Figs. 1, 2, 3, 5, 10, 

 11, 12, we see either the actinal or abactinal side of the Starfish. We 

 look in one case at the water-tube (to) upon which is developed the 

 abactinal system; while in the other profile, drawn from the opposite 

 side, we see the water-tube {w') which develops the actinal system ; the 

 two water-tubes are placed on different sides of the stomach, and have 

 no connection whatever at this extremity, but are separated by the Avhole 

 diameter of the stomach, over parts of which these tubes have spread 

 like a cap. It will at once be noticed that, in any of these figures, each 

 side of the future Starfish makes an independent open curve ; these curves 

 form what appears to us, when seen from the profile view, part of a cir- 

 cular arc. On looking, however, at the same sides from the ventral or 

 dorsal view of the larvse, as in PI. IV. Figs. 1, 2, or the corresponding views 

 of PI. V. Figs. 4, 6, 7, 8, 9, 13, 14, we do not see the arc formed by these 

 sides projected as a simple straight line, as it would be were it all con- 

 tained in one plane. The extremities of the arc, both of the actinal and 

 abactinal area, — that is, the two ends of it which are nearest, one to the 

 water-pore, and the other to the anus, — are seen, as in PI. V. Figs. 1, 

 2, 3, 5, 10, 11, 12, one on one side of an axis passing through the centre 

 of symmetry of the Brachiolaria, and the other on the other side. The 

 only curve which fulfils the conditions of such a projection is that of a 

 Avarped spiral, so that, in reality, when passing (in PI. V. Fig. 10) from 

 r"i, along the edge of the disk, to r'^, r"^, r'i', r"^, 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 r'^, the arm 

 placed nearest the anus, along the edge of the abactinal area, to r'i, the 

 arm next to the water-pore (b). It is the same for the actinal arc, 

 Avhich forms a spiral identical to that of the abactinal area, only bent in 

 the opposite direction. 



The actinal and abactinal regions are, in reality, two Avarpod 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 



