MUSEUM OF COMPAEATIVE ZOOLOGY. 19 



The genitals never leave, or are pushed from, the body of the star- 

 fish, but as the complexity of their reticulation increases they fill al- 

 most the whole space of the interradius between the radially situated 

 terminals. 



The genital which occupies the interradius in which the brachiolarian 

 notch lies, differs from the others in possessing an indentation on one 

 side which is perforated by the madreporic opening. This failure of cal- 

 cification is brought about by the growth of spicular extensions from 

 the edge of the genital which lies contiguous to the madreporic open- 

 ing, but is marked in no other conspicuous manner. 



It will be seen, on a comparison of my description of the way the ac- 

 tinal plates of the starfish form with that given by A. Agassiz,* that 

 there is a difference in our accounts of the growth of these structures. 

 It would seem doubtful that so great a difference could be the result 

 of our studying different species or genera. According to Agassiz,* 

 the large clusters of calcareous deposits which I suppose to be the ter- 

 minals " unite along the edge of the rays, forming a continuous net- 

 work," and they are figured with such a union in Plate VI. fig. 10.* 

 It does not appear that he considers those plates which lie in the angle 

 of the rays as joining with the ray plates, or terminals, although re- 

 maining distinct from each other ; for, later, he says that the limestone 

 deposits in the angles of the rays do not unite laterally. 



It would seem exceptional to suppose that the terminals do join or 

 unite at their edges, and that the first interradial plates, or genitals, 

 unite with them. Such a consolidation would prevent, for a time at 

 least, any subsequent growth of the arms, unless we suppose a resorp- 

 tion to take place. The plates simply interdigitate with each other in 

 Asterias, and there is no union, temporary or permanent, between termi- 

 nals and interradially situated plates or genitals. It is extremely diffi- 

 cult to distinguish the boundaries of the terminals and genitals in live 

 specimens, and it was only by the use of alcohol and some clarifying re- 

 agent that I w'as able to make cut the separation of the two. 



Interamhulacral Plates. — The interarabulaci'al plates, ad, originate 

 after the corresponding ambulacral rafters, as separate calcifications 

 between the lateral t ends of successive ambulacrals. In their early 



* "North American Starfishes," pp. 46, 48. In my references to A. Agassiz's 

 observations on the embryology of the starfish I have quoted from this paper 

 (Mus. Comp. Zool., Vol. VI.). This reprint contains valuable references to the 

 work of other observers made since the paper was first published. 



t Most distantly removed from the median radius. 



