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



OphiuridcB'j they lie at the oral ends of the ambulacra in front of 

 the pair of ambulacral plates, so that the nervous cord and ambu- 

 lacral canal pass downwards between them and the first ambula- 

 cral plates. These also would appear to be parts of the oral ring, 

 although their position is peculiar. The nervous circle of the 

 Asterida is also placed beneath the oral angles at the circum- 

 ference of the membranous oral disc, where likewise there is a 

 groove ; it lies upon the oral disc under the angles, and may be 

 immediately discovered from without, by breaking them off. The 

 circular canal of the ambulacral vessels has the same position as 

 in the Ophiuridce. 



The torus angularis is absent in the Asteridce; the angles 

 themselves consist of a pair of inter- ambulacral plates, — the most 

 anterior pair of adambulacral plates, in fact, — which are applied 

 together to form an angle. Between every pair of ambulacra we 

 observe upon the inner surface an azygos plate, which cannot be 

 enumerated among the intermediate inter-ambulacral plates, and 

 is therefore hardly to be compared with the inter-ambulacral 

 discs on the ventral perisoma of the Ophiurida. 



It appears to me to be exceedingly probable that the parts of the 

 oral ring of the Ophiuridce, here described, are the same as those 

 which constitute the calcareous ring in the Holothuriadce ; those 

 parts of the ring which lie in the direction of the radii to which 

 the longitudinal muscles of the Holothuriadce are affixed, and over 

 which the branches of the circular canal pass to the ambulacra, 

 being absent in the Ophiuridce. The peristomial plates of the 

 Ophiurida are then the analogues of those portions of the lantern 

 of the Sea-urchins to which the alveoli are fixed. 



The nervous cord lies invariably beneath the perisoma of the 

 mouth and the oral angles ; in Holothuria, under the perisoma 

 of the mouth ; in Echinus, beneath the perisoma of the mouth 

 where it is continuous with the ambulacra; in the Asteridce 

 and Ophiurida also beneath the oral angles of the calcified 

 perisoma. The nervous ring lies invariably close to the proper 

 mouth; where there is a membranous oral disc, at its circum- 

 ference, and always above the oral calcareous ring when this exists. 

 The circular canal of the ambulacral vessels lies more or less 

 deeply below the calcareous ring when this is present ; in the 

 Echinida the dental apparatus lies between the nervous ring and 

 the circular canal. 



The relations of the oral ring of the Ophiuridce are perfectly 

 different from those of the buccal plates of the Sea-urchins 

 which cover the external surface of the oral membrane, as in the 

 Spatangida^, Echinoneus, and the regular Sea-urchins. These 



* In the SpatangidcB the mouth lies excentrically to the excavation of 

 the corona and to the circular canal which surrounds its edge, and is close 



