MUSEUM OF COMPARATIVE ZOOLOGY. 35 



The calcification of the stone canal is not treated by other naturalists 

 who have studied the early form of these organs in younger stages of 

 Asteroidea ; so that it is not possible to compare my observation with 

 others as far as this structure is concerned. 



The growth of the calcifications of the pedicellariae in Asterias is the 

 same, or nearly the same, as has already been recorded by A. Agassiz. 

 No histological studies were made of these organs, and my attention 

 was not turned to their minute anatomy. 



A. Agassiz found that the madreporic opening is placed on the 

 " actinal side in the angle between two rays," * and that it is protected 

 by a thick funnel-shaped projection. There seems to be a difference in 

 the position of this opening in some of my larva? from those which he 

 studied, for in the younger larvae of Asterias the madreporic opening 

 does not lie on the actinal side of the larva, even in considerably ad- 

 vanced stages. The position of the madreporic opening is of course a 

 most interesting thing in morphological studies of the young starfishes, 

 and more observations as to its position are needed. 



In the figure already referred to which Metschnikoff gives of the 

 abactinal side of an unknown starfish, the madreporic body appears on 

 the very margin of the disk of the starfish, and would seem intermedi- 

 ate in position between a madreporic body on the actinal and on the 

 abactinal surface. In Ludwig's figure (fig. 94) of the young Asterina 

 in which a stellate form has been taken on, the madreporite (P) is 

 abactinal. In some of my older stages I was unable to discover the 

 madreporic plate, but believe it in all cases abactinal. 



5. Homology of the Plates of Asterias with those of 



Amphiura. 



Of all the Ophiurans the growth of the plates is best known in a 

 viviparous genus, Amphiura squamata, Sars. While it is desirable to 

 know more of the sequence and method of formation of the plates in 

 an Ophiuran which has a development through a pluteus, the genera 

 which present this condition have as yet not been much studied as far 

 as the growth of plates is concerned, and our knowledge of the Ophi- 

 uran plate development wholly relates to Amphiura.f In comparisons 



* Op. cit., p. 45. 



t The author is not of the opinion that any very great exception to the law of 

 the growth of new plates is brought about by what is called abbreviated develop- 

 ment in Echinoderms. In essential points the growth of the calcareous plates in 

 Asterias and Leptasterias, genera representing two types of development, is the 



