198 



HOUEKT TRACY JACKSON ON ECHINI. 



order a new part appears, auricles, which arise from the ambulacra as separate pieces, and to 

 them the retractor muscles are attached. In the Holectypina apophyses and auricles are both 

 existent. In the Clypeastrina, apophyses are wanting, but auricles are developed, and to 

 them protractor and retractor muscles are attached (radial compass muscles are wanting). 

 Further, the auricles in this suborder may arise from the ambulacra, as in the Centrechinoida, 

 or may, as Lovdn showed, be transferred to the interambulacral plates occupying an interradial 

 instead of a radial position. In the Spatangina in adults the perignathic girdle is wanting as 

 the lantern is absent. In the young of Echinoneus, however, as shown by Mr. Agassiz (1909) 

 and Westergren (1911) auricles are developed on the ambulacra as separate styles, similar to 

 those in the Centrechinoida, as in Arbacia (text-fig. 227). Mr. Agassiz (1883, p. 50) has shown 

 that there are small knobs on the interior of the test in the spatangoid Conolampas sigsbei 

 A. Agassiz. These knobs are on the base of the ambulacra and appear to be remnants of auricles 

 which typically occupy this position. Not uncommonly in the Spatangina a process with a 

 suture at its base arises from the interior of an ambulacral plate near the peristomal border. 

 Such processes may be considered cases in which an auricle is retained as a reversionary variation. 

 It is seen that the perignathic girdle presents characters that differ very definitely in the 

 several larger groups of Echini, so that this structure as well as the Aristotle's lantern yields 

 characters of high interest and importance in the comparative morphology and systematic 

 classification of the Echinoidea. 



