ARISTOTLE'S LANTERN AND PERIGNATHIC GIRDLE. 195 



In Phormosoma (text-fig. 22G) the primordial interambulacral plate is retained in the adult 

 in the basicoronal row (text-fig. 43, p. 80), so that the apophysis consists of the upward growth 

 of one plate, instead of two plates as in Centrechinus ; therefore there is no median suture. 

 The apophysis is low and bears the radial compass and rather slender protractor muscles. 

 The auricles are exceptional in that each consists of two pieces joined by suture instead of one 

 piece, as in all other families of the order. The retractor muscles are inserted on the upper 

 of these two pieces. Another striking character of the auricles is that muscles extend from them 

 radially in two directions. The radial peristomal muscles extend inward, and the radial somatic 

 muscles outward, being inserted on the plates of the peristome and corona respectively. Such 

 muscles have been described in Asthenosoma (Loven, 1892, p. 45). Bell (1889, p. 438) says 

 that these radial or " Langsmuskeln " are altogether wanting in Phormosoma, but he is quite 

 mistaken. 



In Salenia (Plate 4, figs. 4, 5) the apophyses are low and on them are inserted the protractor 

 and radial compass muscles. The auricles are slender, not meeting in an arch, and bear the 

 retractor muscles. This structure as well as the lantern of Salenia is entirely characteristic 

 of its order, not showing any features of the Cidaridae with which the Saleniidae were formerly 

 associated (p. 186). 



In Arbacia lixula (text-fig. 227) the primordial interambulacral plate exists in the basi- 

 coronal row, and the apophysis of this single plate is so slight that it can hardly be considered 

 more than a thickening of the same. To it the protractor and radial compass muscles are 

 attached, as usual. The auricles are simple isolated columns standing quite free and bearing 

 the retractor muscles. In Arbacia nigra (text-fig. 228) the auricles may join in an arch over the 

 ambulacrum, though frequently they do not join, and there is considerable diversity in this 

 character in different specimens or in different areas of the same specimen. This joining of 

 the auricles in an arch is a progressive character and A. nigra has other progressive features, 

 as previously discussed (p. 116), marking it as a specialized species in its genus. 



Strongylocentrotus drobachiensis (text-fig. 229) has low but well marked apophyses, to which 

 are attached the protractor and radial compass muscles. The auricles are arched, meet in a 

 median suture, and bear the retractor muscles. In young specimens the auricles are separate. 

 As a matter of individual variation, in adults occasionally the auricles do not join in an arch, 

 but retain the youthful and more primitive character of separate pillars. 



In Echinometra lucunter (text-fig. 230) the perignathic girdle is a very striking structure. 

 The apophyses are high, strongly developed, and bear powerful protractor and compass muscles. 

 The auricles are thick, solid, extraordinarily high, and meet as spoon-shaped plates, arching 

 above the ambulacrum, with a long median suture. The retractor muscles are situated high 

 up on the auricles and are very powerful. 



Loven (1892) gives the characters of the perignathic girdle in Discoidea and Galerites 



