Prof. J. Miiller on the Structure of the Echinoderms. 121 



plates up to the mouth, so that even the series of feet are unin- 

 terruptedly continuous to the same extent. While in Echinus 

 there is only a single pair of oral suckers between the corona and 

 the mouth in the line of the ambulacrum, the series of moveable 

 ambulacral plates of Cidaris terminate above the dental appa- 

 ratus in the form of five lobes surrounding the mouth ; they are 

 distinguished from the angles of the mouth of the OphiuridcB by 

 being ambulacral or radial, while those of the Ophiuridce are 

 inter-radial. The dental apparatus of the Sea-urchins, therefore, 

 is something peculiar which is not possessed by the Ophiuridce, 



There is in the latter an analogue of the calcareous ring of 

 the Holothuriadce which has hitherto been unnoticed, lying be- 

 neath the most anterior ambulacral plates and the oral angles, 

 and affording a basis for further comparison. These parts become 

 visible in an Ophioderma, Ophiocoma, &c., or in an Astrophyton, 

 if the internal surface of the ambulacral skeleton, — that which is 

 turned towards the abdominal cavity, — be examined. Here also 

 we observe the nervous ring and the circular canal of the am- 

 bulacra ; the ambulacral nerve and vessel pass at the oral end 

 of the ambulacrum from above downwards over the most anterior 

 ambulacral plates, so that they appear upon the under surface 

 in the midst of the cleft of the anterior ambulacral plates ; here 

 each enters its ring. The nervous ring lies in a groove, which 

 is excavated transversely upon the inner surface of the united 

 plates of the oral angle. This groove, which is readily visible in all 

 Ophiurida, is covered by the ipecnliar peristomial calcareous plates 

 with which we are now concerned. The membranous ring which 

 constitutes the proper mouth of the Ophiuridce below the oral 

 angles is strengthened at its circumference by these calcareous 

 plates. As a rule, there are ten calcareous plates, which in the 

 Ophiuridce, however, constitute no complete ring ; they are con- 

 joined in pairs and lie upon the lower surface of the oral angles. 

 In Ophioderma two additional plates occur where these two 

 plates meet, one in front of and the other behind their junction. 

 In Ophiolepis ciliata these peristomial plates are least obvious 

 and may readily be overlooked ; but, on the other hand, the cir- 

 cular canal of the ambulacral vessels is here most readily visible, 

 and may be injected or inflated from the Polian vesicles. 



The latter are disposed inter-radially close to the ab-oral edge 

 of the peristomial plates. From the circular canal ten branches 

 pass through little perforations of the most anterior ambulacral 

 plates to the lower oral suckers, which are situated in the stellate 

 cleft above the membranous oral disc. 



In Astropkyton the two plates which lie upon the lower surface 

 of the oral angle are united into a single one, but five additional 

 azygos plates make their appearance which are absent in the 



