HOMOLOGIES AND AFFINITIES OF ECHINI. 



The general symmetry of Echinoderms has been fully discussed by Agassi/ 

 and Midler, and I am only repeating here facts which they have discussed 

 from a somewhat different point of view. Their relation to Radiates is 

 maintained by the former, while Muller considers the plan of radiation as 

 entirely subordinate to that of bilateral symmetry, basing his arguments 

 mainly on the structure of the plan of the pluteus ; Agassiz, on the contrary, 

 drawing his arguments entirely from the fully developed Echinoderm. There 

 is no doubt that in the pluteus the bilateral symmetry completely overshadows 

 the radiate plan, which does not become prominent till after the Echinoderm 

 has passed through the pluteus stage, but then the plan of radiation com- 

 pletely overshadows the bilateral symmetry, which becomes only a secondary 

 feature. Both Agassiz and Muller determine an anterior axis by means of 

 the position of the madreporic body. I have already shown that this method 

 did not lead to accurate results, and that in the regular Echini we bad as yet 

 found no means of determining an axis which should in all cases be homolo- 

 gous to the anterior axis of the Clypeastroids and Petalosticha. In the 

 Echinometradae alone we find an apparent exception to this rule ; yet when 

 we compare Echinometra and Heteroeentrotus we find, as I have pointed out, 

 that the long and short axis, as determined by the position of the madreporic 

 body, are not homologous in these two genera, although so closely allied. 



The embryo Echinoderm only in its earliest stages, after the resorption of 

 the pluteus, creeps upon the same surface whether it be a Starfish, Ophiuran, 

 Echinus, or Holothurian. There is, very early in the life of many Holo- 

 thurians, a distinct dorsal and ventral side, which serves as the sole upon 

 which they creep, while in the Starfishes, Ophiurans, and the greater 

 part of the Echini, the actinal surface remains always the one upon which 

 the animal moves. In the Holothurians, as is well known, we have either 

 apodous genera, or genera in Avhich there are ambulacral feet in all the am- 

 bulacra, only more powerfully developed in the trivium which serves for 

 creeping; or else the ambulacral feet are present only in the trivium, as in 



