690 JAWS - 



The auricles are interambulacral processes ; they are developed from the 

 test itself and do not belong to the dental system, as stated by Loven, while 

 the teeth and jaws are developed independently as isolated pieces in young 

 Echini (PL IX. f. 2)- In the Cidaridae the processes of the adjoining inter- 

 ambulacral auricles are closely connected, and appear to be more intimately 

 related than in the Desmosticha proper, where the interambulacral processes 

 on each side of the intervening ambulacra] space form an arch, which may 

 or may not be closed, and of which the extremity is more or less closely 

 soldered together (compare PL IP. f. .;. 5, Dorocidaris, to PI. IP. f. 9, 10, 

 Diadema). The last trace of what is evidently the hoinologue of the auricu- 

 lar processes of the Desmosticha and Clypeastroids is found in the inter- 

 ambulacral protuberances existing round the actinostome on the inner side 

 of the test of the Nucleolidae ( PL AAA//./. .; ; PL XXXIII f. l). 



The microscopical structure of the jaws has been shown by Valentin to be 

 similar to that of the other parts of the test. The teeth, however, are com- 

 posed of long, slender, narrow plates, composed in their turn of long, slender 

 prisms, slightly curved at the extremities, while the outer sheath is formed 

 of limestone plates, arranged irregularly between and across the component 

 prisms of the teeth proper. The histological components of the muscular 

 tissue of Echinoderms is but little known ; according to Leydig* the muscu- 

 lar fibres consist of rather large wedge-shaped bodies, closely packed to- 

 gether, and surrounded by a delicate membrane. Kollikert has described the 

 muscles of the jaws of Echini as made up of needles of transversely striated 

 muscular fibres. It is remarkable that the muscular structure of the Ophi- 

 urans and starfishes should be so different from that of the Echini, if the ob- 

 servations of Schwalbe % are correct. 



* Archiv. f. Anat. a. Hivs., 1854. 

 t Wiirzburg Verhandl., 1858, VIJI. 

 t Archiv. t. Microscop. Anat.. 18G9, V. 



