35 



The axial organ arises as an epithelial thickening 

 upon one of the primary mesenteries — the longitudinal 

 accessory — formed by the approximation of the cnelomic 

 sacs. Concurrently with the shifting of the vestibule to 

 the posterior end of the larva a re-arrangement of the 

 skeletal plates takes place. 



The five orals (PI. YII., fig. 66, or. p.) form a pyramid 

 on the roof of the vestibule, while the five basals (has. p.) 

 form a similar but inverted pyramid in the body wall of 

 the calyx. The plates forming both pyramids are inter- 

 radial in position. Around the uppermost joints of the 

 stalk there are from three to five small infrabasals, which 

 are said to eventually fuse with the centro-dorsal. During 

 subsequent development of the larva the definitive mouth 

 is formed by perforation of the vestibule, and radial 

 grooves soon divide the vestibular roof into five interradial 

 valves, in each of which lies an oral plate. Between these 

 the five groups of primary tentacles and five primary 

 sacculi make their appearance (fig. 66). The superficial 

 oral nervous system appears as a multilaminar ring of 

 ectoderm, the deeper cells of which give rise to the nerve 

 tissue. The anus, in the formation of which the ectoderm 

 takes no part, breaks through in the posterior interradius, 

 in which also the primary ciliated funnel (hydropore) 

 appears (fig. 68, Jnjp.) The chambered organ is now 

 shut off from the aboral coelom, and the axial organ 

 becomes an independent strand of cells, at first solid, 

 but subsequently hollow. The horse-shoe-shaped hydrocoel 

 (fig. 68, hyd. r.) becomes completely closed and forms the 

 circum-oral water vessel. Four new ciliated funnels and 

 four water tubes (stone canals) make their appearance, so 

 that there is one of each in all five interradii. The larva 

 now consists of a well-marked stalk, composed of from 

 eight to fen cylindrical joints, and a more or less expanded 



