XVI 



ECHINODERMATA 



489 



a quadrangular calcareous frame is made. When the posterior 

 vacuolated crest is formed the anus becomes displaced on to the 

 ventral surface of the larva, and the whole alimentary canal becomes 

 curved so as to be 

 concave ventrally as in 

 the Eipinnaria larva. 

 The left coelomic sac 

 sends up a vertical out- 

 growth towards the 

 dorsal surface which 

 meets the ectoderm. At 

 this point the ectoderm 

 becomes vacuolated and 

 then perforated, and so 

 the primary madre- 

 poric pore is formed. 

 The outgrowth from the 

 left coelomic sac forms 

 the pore-canal. 



Whilst these changes 

 are going on another 

 pair of larval arms make 

 their appearance. These 

 are situated at the 

 sides of the mouth, just 

 where the lateral portion 

 of the ciliated band 

 passes into the anterior 

 cross-bar. They owe 

 their origin to the 

 stimulus provided by 

 the growth of the antero- 

 lateral rod and are 

 termed the antero- 

 lateral arms. There are 

 no appendages in the 

 Bipinnaria which exactly 

 correspond to these in 

 position. 



The larva, not of 

 Ophiothrix fragilis, but 

 of the genus Ophiura, 

 in the four -armed con- 

 dition, was seen by 

 Johannes Miiller (1845). He thought it a new form of adult 

 marine animal, and named it Pluteus paradoxus, from the resemblance 

 it presented to a painter's easel (pluteus) on four legs, when turned 

 upside down. Miiller, as a matter of fact, orientated the larva 



