470 



OPHIUROID PL UTE US. 



rays, and new plates are always added between the ultimate and pen- 

 ultimate 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 



# ^^ y^:/^^^ \\ 



9' d 



FIG. 268. 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. auterointernal arms; e'. posterior arms; g'. anterolateral 



arms ; g. auteroexternal arms. 



stomach is 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 Brachio- 

 laria, but at a certain period the appendages are absorbed. The calcareous 

 rods of the larval arms break up, the arms and anal lobe become absorbed, 

 and the dorsal and ventral discs, with the intervening stomach and other 

 organs, are alone left. After this the discs fit together, and there is thus 

 formed a complete young Ophiuroid. 



The whole of the internal organs of the larva (except the anus), 

 including the mouth, oesophagus, the body cavity, etc. are carried on 

 directly into the adult. 



The larval skeleton is, as above stated, absorbed. 



The viviparous larva of Amphiura squamata does not differ very 

 greatly from the larvse with very imperfect arras. It does not develop a 

 distinct ciliated band, and the provisional skeleton is very imperfect. The 

 absence of these parts, as well as of the anus, mentioned on p. 458, may pro- 

 bably be correlated with the viviparous habits of the larva. With reference 

 to the passage of this larva into the adult there is practically nothing to 

 add to what has just been stated. When the development of the adult is 



