564 



OPHIUROID PLUTEUS. 



There is formed (fig. 262) on the right and dorsal side of stomach the 

 abactinal disc supported by calcareous plates, at first only five in number 

 and radial in position 1 . The disc is at first not symmetrical, but becomes so 

 at the time of the resorption of the larval arms. It grows out into five 

 processes the five future rays. The original five radial plates remain as the 

 terminal segments of the adult rays, and new plates are always added 

 between the ultimate and penultimate plate (Miiller), though it is probable 

 that in the later stages fresh plates are added in the disc. 



The ventral surface of the permanent Ophiuroid is formed by the concave 

 surface between the mouth and anus. Between this and the stomach is 



FIG. 263. DIAGRAMMATIC FIGURES SHEWING THE EVOLUTION OF ECHINOID 

 PLUTEI. (Copied from Miiller.) The calcareous skeleton is not represented. E. 

 Pluteus of Spatangus. 



m. mouth; an. anus; d. anterior arms; d' '. point where lateral arms arise in the 

 Ophiuroid Pluteus; e. anterointernal arms; <'. posterior arms; g'. anterolateral arms; 

 g. anteroexternal arms. 



situated the water-vascular ring. It is at first not closed, but is horseshoe- 

 shaped, with five blind appendages (fig. 262). It eventually grows round 

 the oesophagus, which, together with the larval mouth, is retained in the 

 adult. The five blind appendages become themselves lobed in the same 

 way as in Asterias, and grow out along the five arms of the disc and become 

 the radial canals and tentacles. All these parts of the water-vascular system 

 are of course covered by skin, and probably also surrounded by mesoblast 

 cells, in which at a later period the calcareous plates which lie ventral to the 

 radial canal are formed. The larval anus disappears. As long as the larval 

 appendages are not absorbed the ventral and dorsal discs of the permanent 

 Ophiuroid fit as little as in the case of the Brachiolaria, but at a certain 

 period the appendages are absorbed. The calcareous rods of the larval arms 



1 Whether interradial plates are developed as in Asterias is not clear. They seem 

 to be found in Ophiopholis bellis, Agassiz, but have not been recognised in other 

 forms {vide Carpenter, No. 548, p. 369). 



