HOMOLOGIES AND AFFINITIES OF ECHINI. 759 



interambulacral plates are developed, as in Echini and Starfishes. Of course 

 it is immaterial if there are subsequently ambulacra! suckers piercing through 

 the actinal system, as is the case in young Cuviera ; the homology remains 

 the same. If the general homologies of Echinoderms as thus sketched out 

 are correct, we should reverse the position usually assigned to Holothurians. 

 and place them lowest in the scale of Echinoderms, in spite of their apparent 

 similarity to Worms, which has undoubtedly been the main reason for placing 

 them at the head of the Echinoderms, to which position we would remove the 

 Echini. The position of the Crinoids I should assume to be intermediate 

 between the Holothurians and the Starfishes or Ophiurans. The position of the 

 Holothurians is indeed very striking. They are undoubtedly most closely 

 related to the Echini, yet in one case (Echini) the specialization of the Echino- 

 derm is carried to its maximum degree, as far as the test is concerned ; while 

 in the other (Holothurians) they retain the internal structure of Echini, and 

 yet recall the typical embryonic condition of Echinoderms after leaving the 

 pluteus. Loven has advanced a very similar view of the homologies of 

 Echinoderms, and he has disconnected, as has been done in this chapter, the 

 so-called coronal plates (test) from the ambulacra! and alimentary systems. 

 This has led him to some very ingenious homologies, which we are hardly 

 able to follow in his resume without the further additional details promised 

 for a future memoir. 



The key to the true homology of the Echini and Starfishes is given by the 

 ocular plate of the Echini, first seen by Agassiz, in which the hoinologue of 

 the odd terminal tentacle of the starfish, with its ocular speck (Ehrenberg), 

 is situated. I have shown, in my Embryology of the Starfish, that the new 

 ambulacral suckers and ambulacra! plates are added in Echini and Starfishes 

 between the pointed terminal sucker and the last-formed pair ; new pairs 

 of plates and suckers are always formed at the base of the odd terminal 

 tentacle. The attempt which has been made to homologize the marginal 

 plates of Starfishes in such genera as Astropecten, Astrogonium. Asteriscus, 

 and the like, with the second and third set of interambulacral plates of Palae- 

 chinus, Archeocidaris, and Perischodomus, is more in accordance with the 

 views of the homologies we have advanced ; though this view can scarcely 

 be considered tenable on the supposition that all the plates of the Perischoe- 

 chinidae are homologous, and that the marginal plates of the Starfish genera 

 just mentioned belong entirely to the abactinal system, and have nothing to 

 do homologically with the interambulacral region. 



