1 62 Papers from the Marine Biological Laboratory at Tortugas. 



condition comparable to that of the adults of Chcetodiadema pallidum A. 

 Agassiz and Clark and Centrostephanus rodgersi A. Agassiz. Later still, in 

 Centrechinus, the auricles develop laminar expansions on their upper border, 

 which in large specimens become extensive. 



In Echinometra lucunter, the perignathic girdle is also very specialized, 

 attaining an extravagant development of the auricles. In the young, 

 however, the auricles exist as simple separate styles. Later they arch 

 over the ambulacra, but without being in contact; still later they join over 

 the ambulacra as delicate arches, a condition comparable to that seen as the 

 adult character in simpler species of the genus. Finally, the auricles take 

 on a great development of the dorsal border, producing the spoon-like arches 

 characteristic of the species. 



As shown and urged by Hyatt from his studies of Cephalopoda, and as 

 I have shown in previous studies, in the present study of Echini, and in 

 plants, development, that is the addition of differential characters, con- 

 tinues more or less markedly throughout the life of the individual, and a 

 study of such development yields facts of value in the structure, the mor- 

 phology, and the phylogenetic relations of the group in hand. 



