22 DR. J. F. GEMMILL ON THE DEVELOPMENT OF 



Formation of Aboral Arm-Budiments. — The aboral surface is now somewhat disc- 

 like in shape, with four or iive shallow indentations at the margin, and one deep 

 notch — the nuchal notch — bounded by two prominent lips or lobes, which are at first 

 a considerable distance apart but afterwards draw closely together. The shallow 

 indentations separate corresponding aboral arm-rudiments as these grow out, but the 

 notch (PI. II. figs. 21-23) is produced in consequence of the sharp leftward flexion of 

 the preoral lobe, and the subsequent rapid reduction of the walls and internal cavity of 

 this lobe (see also on p. 27). In a cleared specimen at this stage, viewed from the 

 aboral side (PI. I. fig. 7), the series of hydrocoele-pouches is seen to commence with 

 pouch I lying underneath the sinistral (larval dorsal) lip or lobe of the notch. But 

 this lip will afterwards be associated with pouch II of the hydrocoele in the formation 

 of ray II of the Starfish. Meantime accordingly it may be designated aboral arm- 

 rudiment No. 2. Since, to begin with, it overlies pouch I, it must undergo a sliding 

 movement in the sinistral direction as viewed from the aboral side. This movement 

 is approximately through an angle of 30 degrees. 



The other lip or lobe bounding the notch is the ventral one, with reference to larval 

 orientation. It must be called arm-rudiment No. 1, since in the end it comes into 

 relation with pouch I of the hydrocoele to form the corresponding ray of the Starfish. 

 Before doing this it performs an angular movement in the same direction as arm- 

 rudiment 2 but of greater extent, since in order to reach its final position it has to 

 travel over the area formerly occupied on the right side by the neck of the preoral lobe. 

 The full range of angular movement undergone by aboral arm-rudiment 1 may be put 

 down as something like 60-65 degrees. 



The shallow indentations, referred to above, separate other less prominent arm- 

 rudiments, which appear to the number of three or four just after fixation has begun. 

 These arm-rudiments are to be enumerated in accordance with the numbering of the 

 hydrocoele-pouches with which they afterwards come into relation. Those which now 

 appear are 3 to 5. They have to undergo a certain amount of angular movement in 

 the same direction as arm-rudiments 1 and 2, but the amount is less than that of 2, 

 and it diminishes as one goes down the series. The remaining arm-rudiments — i. e., 

 6 to 9 — appear in the same sequence as the hydrocoele-pouches with which they are 

 to become associated, and each is a little later than its corresponding pouch. 



Attachment hy the first-formed, Sucker-Feet. — Even after the sucker has atrophied 

 there still remain a number of external changes to be noted before metamorphosis is 

 at an end. Some of these changes consist in the completion of processes already 

 begun, and others in the acquirement of new characters or structures. 



The former category includes : — Formation of pouch IX of the hydrocoele and of 

 aboral arm-rudiments 8 and 9 ; partial filling up of the notch between arm-rudiments 

 1 and 2 to form interradius I/II — the madreporic interradius ; complete disappearance 

 by absorption of the larval sucker and arms ; increase in the size and number of the 

 spines. 



