PHYSIOLOGICAL SERIES. 



their relations to the disc in the lower ; in both cases only 



most general features can be seen. The apical system 



ha* been removed with the genital ring sinus in which it 



,, O.C. A1292C. Brit. 



Hmmann, Jena. Zeitschr., Bd. xxiii. 1889, p. 235. 



RCHINOIDEA. 



D. 2. Part of the body-walls of n Ron-urchin (EcMims esculentus] 

 allowing the oral nerv -.-in. 



The superficial system corresponds in its main features 

 to that of Ophiurids. The oral ring (indicated by black 

 paper) lies around the mouth-opening between the teeth 

 and the pharyngeal wall, separated from the latter by an 

 epinoural canal. Opposite each of the five radii it gives 

 off a cord that leaves the lantern of Aristotle between the 

 pyramids, puses under the arch of the auricula, and runs 

 along the mid-lino of the radius, separated from the inner 

 -urfjico of tho ambulacra! plates by an "epineural canal," 

 and from the general body-cavity by the radial canals 

 of the paeudhrcmal, blood-vascular, and amlmlac.ral systems. 

 Near tho apical pole the cords pass through the test and 

 become lost in the general surface epithelium. From the 

 oral ring a few fine nerves arc given off to the alimentary 

 canal, and from the radial cords arise a series of ambu- 

 lacra! and peripheral nerves. The latter perforate the test 

 and form upon its outer surface an intricate plexus, by 

 which the movements of the spines and pedicollarirc are 

 controlled. The deep oral system (not visible in the 

 |iecimen) in in a reduced condition, and is only present 

 upon the inner surface of the oral ring at the point of 

 origin of the radial cords ; it innervates the masticatory 

 apparatus, and is entirely wanting in agnuthous forms. 



In this specimen parts of the oral systems have 1>< < n 

 exposed, showing the ci mi m -oral position of the nerve-nil^ 

 and its relation to the radial cords. One of the latter with 

 vmbnlacral nerves has been isolated by the removal 

 of the ambulacra! j.lat. , of one radius. The apical sy-tnn 

 has been removed \vitli tin- ^"nital-ring sinus. 

 Hamann, Jena. Zeitschr. Bd. xxi. 1887, p. 119. 



