Fewkes.] 
114 
[Dec. 5, 
tation of the possible function of these bodies as motor organs may 
be modified, the homology of these structures in the young Arbacia 
and the young starfish Asterias seems to me well founded . 1 
In Echinarachnius also we have primary spines corresponding to 
the spatulate spines of Arbacia and the fan-shaped spines of As- 
terias, originating very early in the development of the young sea- 
urchin, as I have already elsewhere shown. These spines never 
take on the spatulate form which they assume in Arbacia, but they 
form before the absorption of the pluteus and while there are but 
five 2 plates around the apical area. Their mode of formation is iden- 
tical with that of the terminal fan-shaped spines of Asterias. They 
originate in both instances as nodules or triangular spicules forming 
the base of the spine from which the shaft grows as a calcareous ex- 
tension. Their trifid shape, their relative position as regards the 
radii, their predominance in size when a little older, are identical in 
Asterias and Echinarachnius. It seems not a mistake to regard 
them homologous. The same is true, as far as we know, in regard 
to the relationship of thes^ early formed spines of Echinarachnius 
and Arbacia. They also are homologous . 3 
Accepting now the homology of these peripheral spines in Ar- 
bacia, Asterias and Echinarachnius, is there any observation to show 
that any one of these have free-swimming larvae after the absorp- 
tion of the brachiolaria or pluteus? I believe there is, and it is just 
in that genus which has the primary spines the most spatulate, and 
the best suited for flappers that we have an observation of great 
importance in this regard. 
I have repeatedly taken, the young Arbacia, free swimming, in 
the stage with spatulate spines. I do not recall a large number of 
Echinarachnii after the absorption of the pluteus as found in the 
nets used in the capture of swimming animals, but I have repeat- 
edly taken the young Arbacia after it has absorbed its pluteus , along 
with free-swimming pelagic organisms. From the mode of fishing 
with the “Muller net” it is of course possible that these young Ar- 
1 The origin of these spatulate bodies in Arbacia has not been traced, nor do we 
know, as already elsewhere stated, the relative homologues of the terminals in the 
young sea-urchin. Discovery in this line of work, however, will not change the fact 
that the relation of the spines to the radii is the same in Arbacia as in Asterias. 
2 I can only make out five ; possibly there are ten. 
3 It may seem as if I have magnified the importance of a self-evident proposition. 
I believe it is of the utmost importance to recognize the fact that the first formed pe- 
ripheral spines in Asterias, Arbacia and Echinarachnius are the same. 
