36 BULLETIN OF THE 



between stellate forms of the two groups, Ophiuraus and Starfishes, I 

 have chosen, therefore, Amphiura on the one side and Asterias on the 

 other as representatives. It would seem as if it were necessary care- 

 fully to compare the stellate forms of Echinoderms before we can pass 

 to others, when the external forms are so varied. It may seem as if 

 the difficulties in a comparison of Ophiurids and Asterids would be 

 small, but even here we find very great differences in opinion as to the 

 homology of certain plates, and a variety of interpretations upon struc- 

 tures of primary origin. While it is not proposed in this paper to go 

 beyond a comparison between stellate Echinoderms, it is believed that 

 a more accurate conception of the relationship of plates is possible than 

 that ordinarily accepted. There is range enough in the modification of 

 plates in Ophiurans and Asteroids to call for the best possible state- 

 ments of their relationship in the two groups. 



The following theses may be stated in a tabular form, to indicate the 

 line of discussion which is to follow. Plates of Asterias and correspond- 

 ing plates of Amphiura are placed side by side. 



Asterias. Amphiura. 



Dorsocentral. Dorsocentral. 



First dorsal ? Radialia. 



Genital. Basal. 



Oral ambulacrals.* SpooD-shaped plates. 



Interbrachial ends of oral ambulacrals. Adambulacrals (1 and 2). 



same. It is not intended to compare Asterias witb any Opbiuran except Amphi- 

 ura. The many problematical questions which have arisen in a comparison of 

 Amphiura and Crinoids are also passed over in silence. My object in this paper 

 is to see if it is possible to arrive at a better idea of the homologies of the stellate 

 Echinoderms. A discussion of the homologies of the plates of either with those 

 of Echinoids, Crinoids, or Ilolothurians is not proposed. Amphiura is chosen 

 for a comparison, for the reason that the development of its plates is better known 

 than those of other Ophiurans. From the statements of those who have written 

 on the development of an Opbiuran from the pluteus, it would seem that there is 

 some difference in the sequence of the plates in Ophiurans with and those without 

 a pluteus, but the amount of difference is yet to be made out. 



In this connection, it seems to me that published statements about Ophiophragmus 

 by Mr. Nachtrieb have an interest. Nachtrieb (Studies, Johns Hopkins University, 

 Vol. IV. No. 2) finds that Ophiophragmus has a " development without pluteus." 

 He says he was able artificially to fertilize Ophiophragmus. This is the first time, 

 I believe, that an Ophiuran without pluteus has been artificially fertilized; and, 

 judging from the statements which he makes, the development of Ophiophragmus 

 must be very peculiar. 



* Unrler this designation I refer to the plates which resemble ambulacral rafters, 

 surrounding the mouth. 



