264 RITTER AND CROCKER 



It is obvious enough that as regards most of the particulars 

 in which the bilaterality of the adult manifests itself, they can 

 have no direct relation to the bilaterality of the larva. This 

 would be so, for example, with reference to the radial muscles, 

 the racemose glands, and the position of the pyloric coeca ; for 

 all these belong exclusively to the adult animal. 



Their peculiar disposition is, however, determined by the in- 

 coming of accessory raj^s ; and since, if our interpretation of 

 this process is right, it is already set up during larval life and 

 metamorphosis, and has direct relation to the symmetry of the 

 larva, it y^ovXd. follow that the whole round of bilateral mani- 

 festations of the adult are directly and indirectly referable to 

 the bilaterality of the larva. And our results would then be in 

 accord with those workers on asterid embryology who, like Goto, 

 insist upon such coincidence of symmetry from the evidence 

 afforded by the metamorphosing larva. 



But probably the most interesting question raised by our ob- 

 servations is that concerning the significance of ray A. The 

 facts are all against its being regarded as an accessory ray be- 

 longing to the same category as those acquired later. Its sin- 

 gleness while the others are always paired, its possession of 

 racemose glands while the others are without them, the method 

 of origin of its dorsal radial muscle as contrasted with those of 

 the others, and, finally, its presence in full size relative to the 

 group of five, when the first pair makes its appearance — all 

 these facts remove it from intimate association with the other 

 accessory rays, and relate it more with what we must regard as 

 the group of original five. 



We might suppose that the original ancestor of Pycnopodia 

 was a six-rayed star like Asterias hcxactes or A. equalis, and 

 put the question aside at that ; and perhaps in the absence of 

 any information whatever about the embryonic history of this 

 or any other echinoderm having more than five rays, this is the 

 best we can do. At the same time, the fact that this is a sixth 

 ray and that it holds a unique relation to the two budding zones 

 makes such a superficial disposition of the question quite un- 

 satisfactory, and even though we cannot expect to reach a full 

 solution of the problem in the present fragmentary stale of our 



