THE DJJVELOPMENT OP OPHIOTHRIX FRAGILIS. 581 



the commencement of the encircling o£ the oesophagus of the 

 larva by the hydrocojle; in a word, by the assumption of a 

 hoop-like form by this organ. This stage is shown in dorsal 

 and ventral views in figs. 13 a and Idb, and in section in 

 figs. 43 a and. 43 b. 



As yet the external symmetry, as shown by the larval arms, 

 is unaffected, but the first lobe of the hydrocoele has extended 

 towards the mid-dorsal line above the stomodfeal portion of 

 the oesophagus, whilst the fifth lobe has begun to pass beneath 

 the oesophagus towards the right. Corresponding to this 

 extension of the dorsal portion of the hydrocoele goes a cor- 

 responding extension of the thickened side of the oesophagus, 

 which constitutes the left half of the adoi-al ciliated band. 

 This pushes the dorsal, thin-walled, scoop-like portion of the 

 oesophagus, which is part of the stomoda3um, over to the right. 

 The beginning of this shifting is shown in fig. 13 h. The 

 outer wall of the left posterior coelom becomes thickened and 

 produced into five elevations, which are the rudiments of the 

 arms of the adult brittle-star. Three of these are shown in 

 fig. 13 a; besides these there are two ventral ones. These arm 

 rudiments consist at first of mere conical thickenings, but 

 each soon acquires a lumen which is an evagination of the 

 coelomic cavity (fig. 43 a, arm 1). This lumen forms the 

 dorsal coelomic canal of the adult arm. The greater proportion 

 of the cells of the thickening seem to be destined to form the 

 plates which ensheath the adult arm, but as I have already 

 intimated, all trace of the adult calcareous structures is lost 

 owing to the method of preservation. 



As the metamorphosis proceeds the dorsal and ventral 

 ends of the hydrocoele extend over to the right. 



Soon the external features are involved, as shown in fig. 14 

 and in figs. 44 a, b, and c (which are sections of a specimen of 

 the same age) ; the left antero-lateral arm is carried over to the 

 right. In fig. 44 a it is seen that the madreporic pore and 

 left anterior coelom have shared in the movement, and have 

 reached the mid-dorsal line ; in fig. 44 b the left half of the 

 adoral ciliated band is seen to have increased tremendously, 



