THE SYSTEM OF PHYLI.Ol'IIORINLE. 



6l 



Edwards' statement concerning Holothuria fiondano, PourtalÈs 

 [11, p. 251] seems to be suggestive of the fact that in tvventy-tentacled 

 Holothurids a supernumerary and the last acquired tentacle stands 

 usually dorsal to the radial canal : thus, he says : " In the 7 cases 

 with more than the normal 20 tentacles there appears a slight 

 tendency to the addition of extra tentacles dorsally." 



3. General consideration regarding the 

 multiplication of tentacles. 



On the basis of the data given above, it seems not unwarranted 

 to conclude that the multiplication of tentacles takes place, at least 

 in the group of the polychirotous Cucumarids, according to a general 

 plan, as follows : — 



All the tentacular canals spring from the radial canal separately 

 and alternately on both sides. In paired radii the first or the 

 proximalmost tentacular canal branches out from the radial canal on 

 the dorsal side ; the second stands opposite to the first, i. c. on 

 the ventral side ; the third is again on the same side as the first, 

 and so on. In the midventral radius the canals usually branch out 

 in a way congruent with those in radii of the right side, /. e. the 

 first, the third, etc. arc on the right, while the second, the fourth, 

 etc. are on the left side. The more distal the branching point, the 

 smaller is the diameter of the canal and the size of the tentacle it 

 belongs to, and the nearer is the position of the tentacle to the 

 median line of radius. 



For the sake of convenience, the tentacles to which the first and 

 second tentacular canals belong, may be called tentacles of the first 

 order, and those in relation with the third and fourth canals, tentacles 

 of the second order, and so on. Thus, when a certain Cucumarid is 

 said to have tentacles of. the first and second orders only, it denotes 

 that there exist either fifteen or twenty tentacles in that form. The 

 so-called " decachirotous " forms, such as Cucumaria and Thyone, 



