THE EVOLUTION THEOKY 



129 



" In the other classes of the Echinodermata the water-vascular 

 system is similarly formed, though with interesting deviations. 

 That of the sea-urchin does not differ greatly from that of the 

 starfish, but in the Holothurians we observe greater diver- 

 gencies. Of the five radial tubes (ambulacralia) only three are 

 developed into organs of loco- 

 motion and furnished with suc- 

 tional discs, while the tube-feet 

 of the other two have become 

 modified into pointed tentacle- 

 like formations. The ' stone- 

 canal ' does not extend to the 

 exterior, but opens into the 

 body - cavity. In the Ophiu- 

 roidea and Crinoidea degenera- 

 tion has gone still further ; 

 for being well capable of loco- 

 motion, owing to the greater 

 mobility of their arms, all the 

 ambulacralia have become modi- 

 fied into ampulla-less organs of 

 touch." 



The other organ-systems show a similar uniformity of 

 structure, but the larva-form, finally, furnishes the most con- 

 vincing proof of the phylogenetic relation of all Echinodermata. 

 They differ from the adult animals by their pronounced bilateral 

 symmetry as well as by their soft gelatinous body. In the sea- 

 urchins and Ophiuroids we find the Pluteus-larva form, in the 

 Holothurians the Auricularia form, and in the Crinoids the 

 Bipinnaria forms. All these diverse larvae may be traced to a 

 common ancestral form which had a trigeminous intestine and 

 a simple circumoral cilia- wreath (fig. 39). 



These various developmental stages of the Echinodermata 

 are of particular interest to the Evolution Theory, because they 

 form the connecting link with another stem, the Vermes. A more 

 highly organized marine form, the famous Balanoglossus, passes 

 through a larval stage which, owing to its extraordinary simi- 

 larity, was once held to be an evolution form of an Echinoderm. 

 The adult worm possesses in its ' proboscis ' an organ similar to 



9 



FIG. 38. WATER-VASCULAR SYSTEM OF A 

 STARFISH. 



