MUSEUM OF COMPARATIVE ZOOLOGY. 31 



renorite is represented in the same ring as the terminals, and in posi- 

 tion is a separate plate from the genitals. The genital which occurs in 

 the same interradius is double. 



If this condition or arrangement is found on more extended observa- 

 tions to be the exact relation, it may lead to much light on the whole 

 question of starfish morphology ; for if the madreporite is not a genital, 

 but a distinct plate, the fact adds strength to the belief that the 

 oral of Amphiura is the homologue of the odontophore, while the 

 basal of Amphiura is the homologue of the genital of Asterias. It may 

 render it necessary for us to regard the madreporite in starfishes as 

 ordinarily described as a consolidation of genital and madreporite, 

 which would somewhat affect accepted homologies. The separate 

 calcification of the stone canal, and the eccentric position of the madre- 

 poric opening as compared with the genital, point to to a compound 

 character of the madreporite. 



A. Agassiz's account of the way in which the ambulacral and inter- 

 ambulacral plates of the arms of starfishes are formed, differs from what 

 I find in Asterias. He says (pp. 91, 92), "In the case of the young star- 

 fish, the radial plates of the abactinal system which form the dorsal part 

 of the arms gradually extend towards the edge of, and down on to the 

 actinal side, enclosing the water system little by little, and finally, as has 

 been described, covering the ambulacral tube, leaving only openings for 

 the passage of the tentacles. ... In the, starfishes, the actinal plates 

 formed by the bridges separating successive pairs of tentacles become 

 resorbed along the central line, the edges forming inwardly by spurs the 

 true ambulacral plates, and the plates which little by little develop so 

 as to form the edge of the arms are likewise formed from the plates 

 originally a part of the abactinal system. Those which are on the out- 

 side of the tentacles become the interambulacral plates, but differ in no 

 way from the plates forming the sides of the arms." * 



If I rightly understand his account, there is considerable difference 

 between the way in which ambulacral and interambulacral plates form 

 in the starfish which I studied, and those which he describes. The 



* It is hard to reconcile this view of the way these plates (ambulacral) form 

 with the figures of them by Krohn and Thomson. Agassiz's figures (PI. VI. 

 fig. 12, PL VII. fig. 1, Embryology of the Starfish) of the plates of the actinal side 

 of the arm differ from those of Krohn of a starfish from Bipennaria, and of Thom- 

 son of A. violaceus. They also differ from mine. The separate ambulacral rafters 

 and oral ambulacrals are not represented, but the actinal calcareous plates are 

 represented as joined together. 



