6lO ECHINODERMATA 



whilst the anterior sac divides into dorsal and ventral halves, 

 connected by a narrow neck. The ventral half soon assames 

 the familiar feature of the hydrocoel (Fig. 287, D^ l.hy), whilst 

 the dorsal half forms an insignificant swelling on the course 

 of the conjoined stone- and pore-canals, which represents the 

 left anterior coelom of the other types ; neither right anterior 

 coelom nor right hydrocoel being developed. The neck of 

 communication between dorsal and ventral halves is, of course, 

 the stone-canal. 



The Dipleurula larva leads a free-swimming life for a period 

 varying from two weeks to two months, and then undergoes 

 metamorphosis into the adult form. The details of this process 

 have been worked out in comparatively few cases ; and ' the 

 species in which they are most thoroughly known is the Asteroid 

 Asterina gibhosa. The development of this species is inter- 

 mediate in character between the embryonic and larval types. 

 The eggs are larger than is usual among Asteroidea, and are 

 filled with a bright orange yolk. The larva differs from the 

 Bipinnaria in the absence of the characteristic ciliated bands and 

 in the very early occlusion of the anus. There is, however, a 

 band of cilia round the edge of the prae-oral lobe, which corre- 

 sponds to portions of the prae-oral and post-oral bands combined 

 of the Bipinnaria.^ 



The larva has a form which may be described as boot-shaped 

 (Figs. 288, 289). The sole of the boot is the great prae-oral 

 lobe, behind which is the mouth. The larva takes little or no 

 food, and completes its metamorphosis in ten to twelve days. It 

 does not swim at the surface, but creeps slowly over the bottom 

 by the aid of the ciliated band mentioned above, while it can also 

 attach itself, using the edges of the prae-oral lobe as a sucker. 



After leading an existence of this kind for seven or eight days 

 it fixes itself permanently by a disc-like prominence, which appears 

 on the anterior surface of the prae-oral lobe within the area sur- 

 rounded by the thickened rim which, as explained above, forms 

 a margin to the prae-oral lobe. The larva then becomes divided 

 by a constriction into a disc and a stalk, and the former is 

 gradually converted into the body of the young Starfish, whilst 



' This is clearly seen by comparing the larva of Asterina gibbosa with a youncr 

 Bipinnaria in which the longitudinal band is as yet undivided. The shape of the 

 prae-oral lobe is practically the same in both. 



