20 THE VOYAGE OF H.M.S. CHALLENGER. 



developed until the appearance of tlie Fibularinge which lead us directly to the Echinan- 

 thidse on the one side, and on the other to the Scutellidse, through such genera as Mor- 

 tonia ; Eddnocyamus being the genus in which the clypeaster-like petaloid nature of the 

 ambulacra first appears. 



From the time of the appearance of Galeropygus, Hyhoclypus, and Pyrina, we 

 can readily trace the systematic connection of genera which lead on the one side 

 to the slightly modified Cassiduloids of the present day, such as Echinoneiis and 

 Echinolampas, the Pyrina types showing evident relationship to the Discoidese on the 

 one side, and the Echinolampadee on the other, while Hyhoclyims may well be regarded 

 as the forerunner of the Ananchytida3 and CoUyritidse, the forms of which are still repre- 

 sented at the present day, and of the Echinobrissinae which have also survived to the 

 present epoch, while from the Toxasteridae and the like we pass to Hemiaster, which 

 may well be considered as the oldest of the Spatanginae proper, allied through Micraster, 

 to the Holasteridse, and to which it is not difficult to trace the relationship of all the 

 living genera of Spatangoids. Should we go back to the earliest groups of Echinoidea the 

 Palseechiuidse, let us see how far they show affinities to Echinids now living. In the first 

 place the whole mode of comjiosition of the test is eminently crinoidal from the great 

 multiplicity of plates. It certainly is interesting to find in these earliest Echinids so close 

 a structural affinity in the arrangement of the interambulacral plates with the interradial 

 plates of Crinoids, showing how far-reaching has been Loven's generalisation in which 

 he compared the apical system of the Echinids to the six primary plates of the calyx of 

 Crinoids. And finally, in a still more general way, we can trace in the embrj'o 

 Echinoderm of the difierent orders, whether it be a Starfish, an Ophiuran, a Sea-urchin, 

 a Holothurian, or a Crinoid, the typical structural features which underlie them all. 



We trace the existence of the earliest crinoidal structures in the persistency of the 

 central plate with its five radial plates in the embryo Echinoderms of all the orders. 

 We follow in the irregular arrangement of the plates of the dorsal surface of Starfishes, 

 in the repetition of the joints of the arm-plates of the Ophiurans, in the great number 

 of interambulacral plates composing the test of the earliest Sea-urchins, and of many 

 Holothurians, modifications of the branching of the arms of the early Crinoids. The 

 somewhat indefinite distinction of the ambulacral and interambulacral plates in the 

 embryos of Echinids, Starfishes, and Ophiurans dates back to the earliest Cystideans, in 

 which the presence of an apical and anal system still further obscures the natiu-e of the 

 areas. Thus it is that structural features which have apparently disappeared reappear 

 again suddenly, seeming to have no connection with the types immediately preceding 

 them, from the peculiar combination of characters which have remained persistent down 

 to that moment. Yet when we come to analyse the individual characters thus combined, 

 we generally are able to trace them all as modifications of structural features indicated 

 in older periods, but combined together, perhaps, in so novel a way as at first to defy 



