DEVELOPMENT OF ASTEK/XA 



6ii 



the latter continually diuiinishes in size, and eventually entirely 

 clisap})ears, when the young Starfish commences to walk about 

 on its podia. The disc becomes bent downwards and to tlie left, 

 so as to make nearly a right angle with the stalk, and the last 

 vestige of the latter springs from the peristome of the Starfish 

 inside the water- vascular ring (Figs. 289, B, C). 



The form of the Starfish is attained principally by the 

 preponderant growth of the left hydrocoel and of the left posterior 

 c(»elom. Both these sacs take on the form of hoops, which, 

 by the meeting of their ends, are converted into rings. The 

 liydrocoel has already grown out into five lobes, which are the 



Fic. 288. — Fully grown larval stages of Askiinn gihhosa. A, fully grown larva ; B, 

 left, and C, right view of a larva seven days old in the begiuuing of tlie meta- 

 morphosis, m, Jlouth ; 1-5, the five lobes of the hydrocoel ; I.-V., the rudiments 

 of the arms. (After Ludwig.) x 45. 



rudiments of the radial water-vascular canals, and the tips of 

 which become the terminal sensory tentacles (Figs. 288, 289, 

 1-5); but now the left posterior coelom grows out into five 

 lobes also, forming a parallel but outer ring. These lobes (Figs. 

 288, 289, I.-Y.) are the rudiments of the arms, which are at 

 first quite independent of those of the radial canals, but 

 gradually, when the larva has attained the age of nine days 

 (Fig. 289, B, C), they become applied to the outgrowths of 

 the hydrocoel. These by this time have developed each two 

 pairs of branches, the rudiments of the first two pairs of tube- 

 feet in each radius. The larval mouth vanishes, and a new 

 mouth is formed on the left side in the centre of the hydrocoel 

 ring, when the metamorphosis is complete. The adult anus is 



