ASTEUIAS PALLIDA. 



267 



learned in an earlier stage. In the specimen fioni which Fig. 38 is taken 

 it also lies close to the axial sinus on the posterior side. This cavity 

 persists to the adult stage, as may l)e seen from Figs. oO, 41-45, & 47. 



4'lie water-vascular ring is formed precisely in the same way as the 

 circular enterocœl, but in some places it remains connected by the 

 plate of cells above mentioned with the anterior enterocdd longer than 

 the latter (Figs. 27 & 39). In most parts the canal thus formed is 

 very small, but in the five radii, i.c\, where it receives the radial water- 

 vascular canal it is rather large (Fig. 3Ü). 



The formation of the two ring canals above described must not 

 be considered as a simple meclianical effect of the formatitjn of the meta- 

 stome, as might be inferred from the descriptions of some preceding 

 authors. As may be seen from the figures already referred to the two 

 ring canals in (piestion are complete some time before the forniati(3n of 

 the metastome. 



In a stage a little later than stage F the w^ater-vascular pouches 

 become 3dobed, at tlie same time that the open curve formed by them 

 approaches more nearly a circle. In stage CI (Fig. 15) we see that the 

 pouches are 5-lobed. Tiie circle is not yet complete, but the plane 

 containing the pouches is now almost perfectly ])a railed to the 

 plane of the aboral disc. In stage H (Fig. l(i) the circle is com])lete 

 and the plane of the water-vascular system is perfectly parallel t(^ that 

 of tlie aboral disc. The lobes of the primary water- vascular pouclies 

 have become tlie ambulacra! feet, with the exception of the terminal 

 lobe which is destined to be the terminal tentacle. The disagreement 

 between the numbers of the arms of the star and <_)f the water- vascular 

 pouches is at once a|)parent from Fig. 16 — disagreement which has been 

 recognized by previous students, and clearly pointed out in particular 

 by Ludwig ['82]. 



