492 INVERTEBEATA chap. 



the body ; they become, however, much longer in other forms of 

 Bipinnaria than the one described. In the Ophiopluteus they are 

 very long in comparison with the size of the body, especially the 

 postero-lateral ones, and are supported by calcareous rods; but in 

 other forms of Ophiopluteus larva they are not so long as in the 

 one selected as type. 



(c) The two anterior coeloms extend into the prae-oral lobe in the 

 Bipinnaria, and fuse there into one anterior cavity ; also the septa 

 dividing anterior from posterior coeloms are formed late, and that on 

 the left side undergoes secondary perforations. In the Ophiopluteus 

 the right and left anterior coeloms remain separate, and the septa 

 dividing them from the right and left posterior coeloms are formed 

 early and are not subsequently perforated. 



(d) A vestige of the right hydrocoele is formed in the normal 

 Ophiopluteus larva, but in the Bipinnaria larva this only occurs as a 

 variation. 



On reading over the above comparison it is obvious that, if the 

 fully grown larvae of Asterias glacialis and of Ophiothrix fragilis 

 were met with for the first time, and were mistaken for adult 

 animals, they would be regarded as genera belonging to the same 

 family. The existence of such a deep-seated fundamental resemblance 

 points to the former existence of a simply organized, bilaterally 

 symmetrical, free-swimming, common ancestor of Asteroidea and 

 Ophiuroidea. Our faith in the former existence of this ancestor will 

 be further strengthened when we have examined the development of 

 the three remaining classes of Echinodermata. 



METAMOEPHOSIS OF OPHIOTHRIX FRAGILIS 



Stage A. — The metamorphosis of Ophiothrix fragilis is initiated, 

 just as is that of Asterias glacialis, by a preponderate growth of the 

 organs of the left side, viz. the left hydrocoele and the left posterior 

 coelom. The former of these grows long and extends forwards, along 

 the oesophagus, completely overlapping the left anterior coelom. At 

 the same time it becomes divided into five lobes by constrictions 

 which appear on its inner side. These are, of course, the rudiments of 

 the five radial water- vascular canals and of the azygous tentacles or 

 tube feet in which they terminate (Fig. 377, C). By this growth the 

 original posterior end of the stone-canal is pulled forwards, so that it lies 

 in front of what was originally the anterior end. A madreporic 

 vesicle is formed, apparently in the same way as in Asteroidea. 



The left posterior coelom also extends forwards. Its inner wall 

 remains thin, but its outer wall begins to develop great proliferations 

 of cells which form conical protuberances. Into each conical pro- 

 tuberance a narrow diverticulum of the lumen extends. These 

 protuberances, which raise corresponding humps in the ectoderm, are 

 the rudiments of the adult arms. 



As the left hydrocoele increases in length its anterior end begins 



