ON THE EMBRYOLOGY OF ECHINODERMS. 21 



the young Starfish, and must ha^■c been formed in a similar way from the original 

 simple loops of fig. 31. (See Proc. Am. Acad., fig. 15.) Additional tentacles are 

 therefore formed* at the base of the odd loop t\ probably in the same manner as in 

 Asteracanthion. The mouth is not limited by the formation of the actinal floor ; this, 

 as well as the abactinal area, is in the present stage almost entirely made up of the 

 remaining portions of the Pluteus, which have not been resorbed, and through which 

 the parts of the young Ophiuran are seen. 



The number of young Ophiuridic observed is not sufficient to enable us to 

 make much use of their earlier condition for a classification. The figures of the 

 young given by Miiller, and the few species of which Liitken has studied some of 

 the earlier forms, together with the observations of Kruyer,* are all we have. The 

 figures of young Astrophi/ton eucnemis M. T., given by Liitken in his Additamenta ad 

 Historiam Ophiuridarum, throw some light on the classification of Ophiuridfc. Hav- 

 ing had the opportunity to examine very young specimens of Astrophytou Agassizii 

 Stimp., collected at Eastport, Me. by Mr. Verrill for the Museum at Cambridge, I was 

 enabled to repeat his observations, and find the same remarkable differences between 

 young and adult which had already been pointed out by Liitken. A young Astro- 

 phyton would seem at first glance to belong rather to Asteronyx or Asteroporpa 

 than to Astrophyton. The disk is circular ; there are no ribs ; the arms have but 

 a sino-le fork. The ribs on the disk make their appearance when each arm has 

 divided three times; that is, when there are twenty terminal points. Up to that 

 period the rounded plates of the disk were quite prominent, somewhat resembling 

 in their arrangement those of the disk of Ophiopholis. This is sufficient to show 

 that the Astrophytidae stand highest among Ophiurans ; that Ophiurans with smooth 

 arms, as Ophiura, are lowest ; next come such genera as Ophioglypha and Amphiura ; 

 Ophiopholis next ; while Ophiocoma, Ophiothrix, and the like, in which the spines 

 take their greatest development, stand highest among Ophiurida; proper. 



The stages represented in figs. 32 and 33 are somewhat different from any given 

 by Miiller ; the nearest conditions are those of fig. 2, Plate VIL and fig. 4, Plate IV. 

 fifth Memoir, in which the outline of the young Ophiuran is far less well defined, 

 but in which the abactinal plates, as well as the arm plates, are further advanced ; 

 the condition of the tentacles is nearly identical. In its general outline fig. 33 differs 

 but little from fig. 30, where the abactinal and actinal floors are more advanced, show- 

 ing otherwise no difierences which would lead us to suppose that the mode of develop- 



« Kroter. Nat. Tidskrift, HI., 1810. 



