298 On the early Development of Antedon rosacea. [Dec. 22, 



before the rupture of the vitelline membrane, and are absent from the 

 ciliated areas. 



The free larva swims with the terminal tuft of cilia directed for- 

 wards. A white patch on its left side between the third and fourth 

 ciliated bands marks the position of the " water-pore." 



Internal Anatomy. — The gastrula has at first no mesoderm, but this 

 soon becomes budded off from the archenteron. The blastopore closes 

 near the posterior end, but whether ventrally or dorsally could not be 

 determined. The archenteron, which only occupies the posterior half 

 of the larva, soon divides into two parts ; the posterior of these 

 (enterocele) assumes the form of a dumb-bell, round the constricted 

 part of which the anterior half (mesenteron) grows till it forms a 

 complete ring. The two swellings of the dumb-bell soon separate to 

 form the right and left body-cavities respectively. From the anterior 

 part of the mesenteron are budded off the hydrocele (left and ventral), 

 and an unpaired anterior body-cavity. 



By a change in position of the right and left body-cavities (incor- 

 rected described by Grdtte), the left body-cavity becomes posterior 

 find ventral, while the right becomes anterior and dorsal : the latter 

 sends a five-chambered prolongation into the praeoral lobe, to form 

 the rudiment of the "chambered organ." The hydrocele forms a 

 ring, incomplete towards the left, on the ventral side of the mesen- 

 teron, and soon forms five ventral pouches. Shortly before fixation, 

 the anterior body-cavity, which extends far into the praeoral lobe, 

 opens to the exterior on the left side by the " water-pore." 



Underneath the anterior tuft of cilia and the praeoral pit, and down 

 the sides of the larval mouth, run fine fibres, which appear to be 

 parts of a larval nervous system which disappears when the larva 

 loses its freedom. 



Fixation and Subsequent Changes. 



After swimming freely for about twenty-four hours, the larva fixes 

 itself by means of the praeoral pit, which forms the disk of attachment, 

 The ciliated bands then disappear, and the larval mouth invaginates 

 to form the vestibule, which is rotated to the posterior end, as de- 

 scribed by Barrois (' Comptes Rendus,' May 24th, 1886). At the 

 same time all the tissues undergo histolysis, and the mesenteron 

 becomes filled with cells budded in from the centre of the hydrocele 

 ring. 



The right and left body-cavities, which are now both dorsal, grow 

 rapidly round to the original ventral side, being separated by a trans- 

 verse mesentery, and each forms a longitudinal mesentery near the 

 original ventral radius. The free end of the larva may be called the 

 oral end, since the mouth now appears as a depression in the floor of 

 the vestibule. 



