97 
[Fewkes. 
plates of starfishes by a study of the development of these struct- 
ures in it as compared with Asterias, than by a study of stellate gen- 
era alone. This study and comparison are particularly desirable as 
far as the oral plates are concerned from the fact that it is supposed 
that the ring of plates surrounding the mouth in this genus ( Aster- 
ina) is said to be formed on another plan from that in Asterias. 
The following observations are published to support certain the- 
oretical considerations which seem to indicate that, although so 
different in form and position, ambulacrals and adambulacrals are 
morphologically the same. It is held that in their origin and in 
younger conditions of their growth they are identical, and that in 
each we have the same structure now modified into an ambulacral 
rafter, now into an adambulacral. We may, in other words, regard 
the skeleton of the starfish arm on the actinal side as composed of a 
number of somites the alternate members of which are calcified so 
as to approach each other and to join on the middle line of the arm. 
It is theoretically supposed that typically inEchinoderms the calci- 
fication of each somite may be thought to surround the water tube 
of the arm, but that in the starfish the ambulacrals have the exter- 
nal half aborted and not represented, while in the adambulacrals 
both external and internal parts of the calcification are missing, the 
lateral portions only of the ring remaining. 
In the sea-urchin, however, the plates corresponding with the in- 
ternal ambulacrals of the starfish are wholly lost, while the lateral 
ends of the interambulacrals remain, and form the external so- 
called ambulacrals. In the mouth plates of both starfishes and sea- 
urchins we have plates, however, more closety approaching the 
theoretical type than any of the plates of the arm. 
It is thought that a study of the mode of development of the 
plates of the body and arms of the pentagonal starfish, Asterina, 
ought to throw some light on the homology of the plates of sea- 
urchins and starfishes. Since we do not know how the plates of 
the sea-urchin form 1 we cannot yet build any very substantial fab- 
ric of homology on embryological grounds, but from what is known 
of the anatomy of the adult starfish and that of the adult sea-ur- 
chin we find many difficulties in our acceptance of the commonly 
taught homologies. 
A primary difficulty in homologizing the ambulacrals of star- 
1 In the young Echinarachnius the plates in this area are very late to form. 
PROCEEDINGS B. S. N. H. VOL. XXIV. 7 MARCH 
