Fewkes.] 
96 
[Dec. 5, 
ON THE SERIAL RELATIONSHIP OF THE AMBULACRAL 
AND ADAMBULACRAL CALCAREOUS PLATES 
OF THE STARFISHES. 
BY J. WALTER FEWKES. 
The skeleton of the actinal region of the starfish is made up of 
two series of paired plates, one of which is called the ambulacral, 
the other the interambulacral, or adambulacral . 1 These plates oc- 
cupy a definite relation to each other, and are ordinarily given a 
different name and a different homology. 
They have, it is true, in adult forms of the starfish very little re- 
semblance in general character, for they differ in shape and position, 
one from the other, and while the members of one set, or series, ex- 
tend across the ray of the arm and join on the median line, those of 
the other are limited to the border of the ray and do not approach 
at this line. One series bears no spines ; the other is spiniferous. 
The members of one series alternate with the legs ; those of the 
other lie on the sides of these organs, and although, at first sight, 
these plates differ so much that they are regarded of different mor- 
phology, they seem to be homologically the same. 
It is the object of the present paper to consider the question of 
the serial relationship of these plates. An answer to this question 
is thought to have a very important bearing on the morphology of 
all Ecliinoderms, especially the homology of the calcareous plates 
which form the ambulacral areas of sea-urchins and starfishes. 
It would appear that the pentagonal starfishes are nearer the Echi- 
noidsthan the stellate genera, and it is possible that we can discuss 
the relationship of the starfishes and sea-urchins better if we know 
more of the manner of development of the calcareous plates in some 
pentagonal genus 2 of Asteroids. Asterina 3 is chosen as such a rep- 
resentative pentagonal genus and we may arrive at a better con- 
ception of the relationship of the ambulacral and interambulacral 
1 I use these two terms interchangeably in this paper. The term interambulacral is 
not only the oldest but is embryologically more accurate. 
2 In the younger form stellate genera are pentagonal, 
3 The rich collecting grounds near the marine laboratory at Roscoff, France, fur- 
nished me abundant material to consider these and some other questions connected 
with the development of Echinoderms and I readily availed myself of an invitation 
to visit there for the consideration of this subject. 
I take pleasure in expressing my thanks to Prof. Lacaze Duthiers for his kindness 
in opening the resources of the laboratory to me and granting me many facilities for 
work. 
