434 MERISTIC VARIATION. [part i. 



In the normal there are 5 radii and interradii, and 10 tentacles : 

 in the abnormals there are 6 and 12 respectively. In half-grown 

 normals the 3 ambulacra of the ventral triviuni have more tube-feet 

 than the 2 ambulacra of the bivium ; also the pair of tentacles corre- 

 sponding to the central radius of the trivium are smaller than the rest. 

 In the abnormals 3 ambulacra have more tube-feet and are separated 

 by narrower interradii than the rest, and of them the central has the 

 least pair of tentacles : therefore these are the 3 radii of the ventral 

 trivium, and of them the central is the central of the normal. The 

 structure of the calcareous ring bears out this correspondence. The 

 central radius of the ventral trivium is therefore not the intercalated 

 radius. 



In the 6-rayed specimens there is thus a ventral trivium and a 

 ' dorsal trivium.' (There were 2 Polian vesicles in 3 specimens, 3 in 

 one and one in the other, but in the normal also these vary in number.) 

 The stone-canal was single in all ; but in one of them it could be seen 

 that the canal arose in the interradius to the left of that which bore the 

 madreporic plate, suggesting that the radius thus crossed was super- 

 numerary ; for in a normal the interradius of the dorsal mesentery is 

 in the centre of the bivium. In a normal there are in the calcareous 

 ring two radials on either side between the dorsal mesentery and the 

 ventral median radius. In 4 of the abnormals (to which alone what 

 follows refers) there were 3 such radii on the left and 2 on the right, 

 while in the 5th specimen there were 3 on the right and 2 on the left. 



The respiratory trees of the normal are in the right interradius 

 of the bivium and in the left interradius of the trivium. In the 6-rayed 

 they are in the left interradius of the ventral trivium and in the lower 

 right interradius of the dorsal trivium, agreeing with the normal and 

 shewing that the right radius of the ventral trivium is not an intercalated 

 one. Next, the mesentery in its course traverses in the 6-rayed form 

 4 radii and 3 interradii, the lower right interradius of the dorsal trivium 

 with its 2 adjacent radii alone being free. In the normal, 3 radii and 

 2 interradii are thus traversed, the right bivial interradius and its 2 

 adjacent radii being free. Therefore the right radius of the dorsal 

 trivium and of the ventral trivium are not intercalated. The central 

 radius of the ventral trivium has already been excluded ; therefore the 

 intercalated segment is either the middle or the left of the dorsal, or 

 the left of the ventral trivium. 



In a normal, the mesentery which is attached to the alimentary 

 canal at that place where its upward portion again turns downwards 

 comes from that interradius which bounds the ventral trivium on the 

 left. This is the case also in the abnormals, and therefore the left 

 radius of the ventral trivium is not intercalated. Of the two remaining 

 radii the left of the dorsal trivium is in nowise abnormal, but the 

 central dorsal radius is abnormal in that it is crossed by the sand-canal, 

 therefore the central dorsal is the intercalated radius. 



And since in four cases there were three radii in the calcareous 

 ring on the left, between the interradius of the stone-canal and the 

 central of the ventral trivium, and two on the right, therefore the new 

 segment is in them intercalated on the left of the median interradius of 

 the bivium ; while in the fifth specimen the intercalation has been made 

 on the right of the same interradius. 



