208 



formed individual settles on the bottom. As in the sea stars and sea urchins, 

 metamorphosis in the brittle stars is catastrophic. The larval organs and. 

 tissues degenerate and are reorganized. The posterolateral arms supporting the 

 juvenile in the plankton remain longer than the other arms. In some species, 

 after the brittle star settles on the bottom, these arms with their fi^inge of 

 ciliated band swim independently in water for some time, similar to the 

 preoral lobe of bipinnariae of astropectinids and luidiids after the conclusion 

 of metamorphosis. 



Skeleton : No amniotic sac forms in brittle stars and the commencement 

 of metamorphosis can be considered coincident with the appearance of pro- 

 cesses in the hydrocoel. However, the first spicules, from which the elements 

 of the definitive skeleton will develop, appear somewhat earlier than the 

 hydrocoelic processes. They are situated on either side of the stomach, with 

 five spicules on each side (Figure 161). With ftirther development, they give 

 rise to five radia' and five terminal plates. The central plate of the aboral side 

 appears somewhat later. Next come the rudiments of the proximal oral plates 



Figure 161: Ophiothrix fragilis (McBride, 1907). 

 Arrangement of different parts of the coelom at commencement of metamorphosis in brittle 



stars. 



A — before turning and reduction; B — after turning and reduction of larval arms; 



axel — left axocoel; axcr — right axocoel; hcl — left hydrocoel; her — right hydrocoel; 



mp — madreporic pore; scl — left somatocoel; scr — right somatocoel. 



