THE DEVELOPMENT OP ASTBRINA GIBBOSA. 229 



thickened ridge with long ciliaj which is the locomotor organ 

 of the larva, and is the first thing to disappear in the meta- 

 morphosis. 



Stage C is the point which we have now reached, and it is 

 characterised by the appearance of this disc for fixation. 

 Ludwig compares the larval organ to the non-ciliated processes 

 of the Asterid larva, the Brachiolaria. This larva appears to 

 be merely a further stage in the development of the well- 

 known Bipinnaria, from which it differs in the development of 

 three stalked papillse from the apex of the prseoral lobe, which 

 are presumably used for attachment. These papillse arise between 

 the anterior dorsal and the anterior ventral arms of the Bipin- 

 naria : one of them is median and more dorsally situated than 

 the other two, and to this arrangement Ludwig compares the 

 occasional bifurcation of the ventral lobe of the larval organ of 

 Asterina. Now, however, that we know the function of the 

 adhesive disc, it is, in all probability, this which is to be com- 

 pared to the papillse of the Brachiolaria; and the larval organ 

 with its long cilia (compare PI. XX, figs. 133 — 135) in all pro- 

 bability represents some portion of the ciliated bands of the 

 Bipinnaria. Garstang (6) has, in fact, recently described a 

 Bipinnaria in which the dorsal arm of the prseoral lobe exe- 

 cutes muscular movements in the same way as Ludwig asserts 

 for the Asterina larva. I repeat, however, that the latter can 

 swim by ciliary action alone, without any muscular move- 

 ment. 



The internal changes which have occurred between Stages B 

 and C are numerous and important. We have already referred 

 to the appearance of the stomodseum or larval oesophagus. 

 About the same time the primary madreporic pore is formed ; 

 it arises by a pocket of the ccelom slightly to the left^ of the 

 mid-dorsal line, meeting a thickening of the ectoderm (fig. 

 26, mp.) and a perforation taking place. The pocket of the 

 ccelom is called the " pore-canal " [pc, fig. 36), and is lined 

 by cylindrical ciliated cells. By this time the two posterior 



1 This position is not shown in fig. 26; the figure represents a section which 

 was rather oblique. 



