I V.LAMOCRINUS DIOMED l 



according to him, is the axis passing from the interambulacrum / to the 

 ambulacrum IV. this is nol an axis which has any morphological value or 

 is characteristic of the pluteus stage of Echinoderms. Such an axis and the 

 one which represents the suture of the spiral in the young Echinoderm is, 

 on the contrary, an axis which corresponds to a line passing through the 

 ambulacrum V and the interambulacrum .'. This will show fully as well 

 as the tallies which 1 gave in the Eteporl on the Echinoidea of the Voyage 

 of the Challenger (pp. 4-8) the artificial character of the axis he has called 

 am, passing through l-IV, and to which he attributes so -real a morpho- 

 logical value, — which is neither greater nor less than thai of any similar 

 axis passing through any of the interarabulacra ■'>. .,'. 5, to their opposite 

 ambulacra I, II, III. 



Therefore, if we wish to adopl a notation for the ambulacral ami inter- 

 ambulacral plates which is to have a morphological character, we must 

 begin (in the Spatangoids) with the ambulacrum V [I]- follow with the 

 interambulacrum 4 U1 '■> ambulacrum IV [II]. interambulacrum 8 [~?] ; 

 ambulacrum III [III], interambulacrum .J [•->']; ambulacrum II [IV]. in- 

 terambulacrum 1 [^] ; ambulacrum I [V], and interambulacrum 5 [-TJ- 

 The new notation in brackets, [I] [.;], being the notation which indicates 

 the primordial axis in the young Echinoderm, while the other, beginning 

 at V and going towards III, corresponding to Loven's notation of V .'. 

 is merely that of the artificial axis aco of Loven. which passes through 

 from 1 to IV (Agassiz [4] [II]). 



Sladen, it seems to me. has entirely misconceived Loven's argument for 

 the notation he has adopted, in defining as I, II, III. IV, V, the ambulacra 

 of Echinoidea. lie has drawn his arguments not from the fact that the 

 madreporic pore indicates in the embryo the line of suture of the spiral 

 abactinal system; but it is from the size of the ambulacral plates adjoining 

 the actinal area that he has fixed his longitudinal axis. His anterior and 

 posterior extremity is originally determined by the presence of a bivium 

 and a trivium in Spatangoids. and then by analog}' its homologue is fixed 

 in the Eueclnnoidea. T have, on the contrary, always insisted that the 

 line of suture of the two ends of the spiral of the abactinal system was 

 the only safe line to use as a guide for determining the axis of the Echini, 

 since I first described the spiral condition of the Starfish embryo.* 



* Embryology of the Starfish, p. 51. 



