120 Prof. J. Miiller on the Structure of the Echinoderms. 



oral angles of the Ophiurida and Euryalida to be considered as 

 alveoli, and are they homologous with the alveoli of the Sea- 

 urchins ? In such case the alveoli of the Sea-urchins, which 

 consist of two halves, and whose angles are also inter-radial in 

 relation to the mouth, must be considered to be metamorphosed 

 inter-ambulacral plates, separated by a great space from the 

 plates of the corona, but connected by muscles with the auri- 

 culae, and these alveolar muscles would be the analogues of the 

 inter-vertebral muscles of the Ophiurida. 



Important considerations, however, are opposed to this inter- 

 pretation. On more close examination, the apparent analogy of 

 the oral angles of the Ophiuridce with the alveoli of the Sea- 

 urchins completely disappears. In fact there exists upon the 

 vertical obtuse edge of the oral angle, both in the Ophiuridce and 

 EuryalidcB, a peculiar azygos plate [torus angularis), upon which 

 the papillse angulares and the dentiform plates are seated. 

 These azygos plates upon the oral angles are wanting in all 

 Asteridce ; they would themselves have a claim to be considered 

 analogous to alveolar plates, if the so-called dental plates of 

 Ophiurida were to be regarded as true teeth. The five azygos 

 alveolar plates of the Ophiuridce in question, however, have no si- 

 milarity with the conjugate alveoli of the Sea-urchins ; nor have 

 the dentiform plates or palm of the Ophiurida more resemblance 

 to the enamelled teeth of the Sea-urchins, since they exhibit the 

 ordinary osseous structure. Furthermore, if we take into con- 

 sideration the manner in which the pala are inserted upon the 

 plates of the oral angles, it clearly results that they are not teeth 

 at all. They are in fact moveable and united with the angular 

 plates by two muscles, which are inserted into deep excavations 

 or perforations lying in pairs in these plates. These perforations 

 in part pierce the torus angularis and extend as far as the bases 

 of the oral angles. The other insertion of the muscles is into 

 the upper edge of the base of the palse, so that when they con- 

 tract, the outer ends of the palse are di-awn upwards. They 

 are doubtless used as manducatory organs. The papillse angu- 

 lares above the palse are also moveable, but they possess no 

 such muscles at their bases; the Euryala have only papillse 

 angulares on the plates of their oral angles, but no palse. The 

 peculiar plates at the oral angles of the Ophiurida are therefore 

 to be regarded as bases of the lips to which the labial papillse or 

 dentes spurii are attached. The comparison of the oral angular 

 plates (tori angulares) or most anterior inter-ambulacral plates of 

 the Ophiurida with the alveoli of the Sea-urchins is therefore 

 incorrect. This results also from other facts observable in the 

 Sea-urchins themselves. In Cidaris, in fact, the corona is con- 

 tinued in the form of moveable ambulacral and inter-ambulacral 



