ECHINODERMATA. 565 



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 larvae with very imperfect arms. 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. 549, may probably 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 fairly 

 advanced the part of the body with the provisional skeleton forms an 

 elongated rod-like process attached to the developing disc. It becomes 

 eventually absorbed. 



Echinoidea. The Echinus larva (fig. 263) has a Pluteus 

 form like that of the Ophiuroids, and in most points, such as the 



FIG. 264. Two LARWE OF SxRONGYLOCENTRUS (From Agassiz.) 

 m. mouth; a. anus; o. oesophagus; d. stomach; c. intestine; v '. and v. ciliated 

 ridges ; iv . water- vascular tube ; r. calcareous rods. 



presence of the anal lobe, the ciliated band, the provisional 

 skeleton, etc., develops in the same manner. The chief difference 

 between the two Pluteus forms concerns the development of the 

 lateral arms. These, which form the most prominent arms in 

 the Ophiuroid Pluteus, are entirely absent in the Echinoid 



