I ILA.M0CRIN1 S DIOMED E 71 



urans, while in the Crinoids they are developed vertically to constitute 

 a Btem or its homologue. In the Holothurians this central system is reduced 

 to a minimum. In the Starfishes and Ophiurans the central system is 

 not limited to a circumscribed area, as in the Echini or Crinoids. In Echini 

 it is limited t<> the anal system, in Crinoids to the stem; in Starfishes and 

 Ophiurans it includes all the plates of the upper and lower surface of the 

 disk which are neither the original genitals nor the terminals, and which 

 do not belong to the ambulacral system. 



This view would restrict the homology of the first formed plates of em- 

 bryo Echinoderms, and leave us to deal with general modes of development 

 diverging very rapidly in very distinct directions from such a generalized 

 Echinoderm embryo as we may recognize. 



The mode of formation of new segments is totally unlike in the arms of 

 Crinoids from the manner in which additional abac tin al plates are formed 

 in Ophiurans and Starfishes. New plates are formed by the forking of the 

 terminal joint, one of the joints developing into a pinnule, the other form- 

 ing the extremity of the arm, and forking again after a time to repeat the 

 same process. 



Although the mode of growth of the ambulacral system is the same in 

 the Ophiurans, Starfishes, Echini, and Crinoids. new tentacles being inter- 

 calated at the base of the primitive terminal ones, yet the position of the 

 terminals in the Ophiurans and Starfishes is very different in the adult 

 from what we find it in the very young stages, while in the Echini it has 

 not materially changed, and there is no plate occupying such a position 

 in the Crinoids. 



In the Crinoids the interradial plates are developed between the orals 

 and the basals, and the basals are as much an interradial plate as any 

 other, even if the name interradial has received a more limited application. 



In 1864, while studying the embryology of Echinoderms.* I discovered 

 that in the young Echinus, soon after the resorption of the pluteus, the 

 anal system was only covered by one large plate, that the anus opened on 

 one side of this, and that smaller additional plates were added at the anal 

 edge; and I subsequently showed that in many genera this original anal 

 plate could still be distinguished in the adult, although its prominence had 

 been greatly diminished by the increase in number and size of the other 

 plates covering the anal system. I compared this anal plate with the sur- 



* Embryology of Echinoderms, Mem. Am. Acad., IX., 1864. 



