1888.] 
113 
IFewkes. 
that there are at least two kinds of spines in asteroids, one kind 
formed as papillae from the plates with which they are solidly con- 
nected from the very first, the other formed from a separate centre 
of calcification which is later articulated with the plate. We must 
either suppose the spine homologous with a plate, or that these two 
kinds of spines are not homologous. It seems to me that the latter 
hypothesis is the better one, and that we can recognize two kinds 
of spines on the external surface of the Asterias. The former are 
the true spines, the latter simply extensions of the plates. Some 
of the former have fan -shaped bodies, others are simple shafts of 
uniform diameter. To the former or movable kind of spines be- 
long those early formed in the young starfish. 
Consider also the character of certain of the first formed spines- 
of the echinoid genus, Arbacia. The first formed spines of this 
genus are paddle-shaped structures totally different in form from 
the permanent spines of the body. They form spatulate bodies 
around the rim of the young Arbacia long before the true spines- 
appear. They are very prominent and can be well seen in the pub- 
lished figures by A. Agassiz and those by Garman and Colton. 1 
The first formed or spatulate spines of Arbacia resemble organs- 
of locomotion both in form, in position and in general character. 
It would seem as if we have here a young echinoid with a likeness- 
to the free swimming ancestor demanded by my hypothesis. 
The spatulate spines of Arbacia are thought to be homologous with 
the ten early formed spines on the terminals of Asterias. Their 
position is the same as regards the abactinal surface, for in the Ar- 
bacia they are situated on the rim of the disk, while in Asterias they 
lie peripherally to the terminals. Moreover they occur in groups 
separated by a radius in which lies the primary water-tube. Call- 
ing this radius the radial axis, of which axes in both genera there 
are five, and the intermediate radii the interradials, each member 
of a pair of the spatulate spines of Arbacia and the fan-shaped 
spine of the terminal of Asterias lies between a radius and an in- 
terradius on the border of the circular disk, which limits the actinal 
and abactinal surfaces of the two genera. However my interpre- 
1 Garman and Colton (Stud. Johns Hopkins Laboratory, vol. 2, p. 253) have called 
attention to the fact that the young Arbacia aids its movements by pushing down- 
ward and laterally with its spatulate spines. They say nothing of the use of these 
organs in swimming. They find these spines cannot be bent upwards “owing to the 
projecting rim at the base of the spine above the obliquely inserted pedicle.” 
PROCEEDINGS B. S. N. H. 
8 
MARCH, 1889. 
VOL. XXIV 
