114 HENRY BURY. 
ancestor was still extremely small and the tentacles few in 
number. 
Next I would point out that the correspondence of the 
series of plates with the body-cavities indicates that their 
symmetrical arrangement arose at a period when the distinc- 
tion between these cavities was still very strongly impressed 
upon the animal. In recent adult Echinoderms this relic of 
bilateral symmetry is almost entirely lost; but in ontogeny, 
as we have seen, it is often retained (with approximate 
equality of the body-cavities) for some time after metamor- 
phosis. 
Lastly, my observations go far to prove that, whatever may 
be true of other skeletal plates, we have in Echinids, Asterids, 
and Ophiurids at least one set—the terminals—which are 
homologous. They are developed over the left body-cavity, 
and in all cases embrace the unpaired tentacles at the ends of 
the radial canals, which may be the reason that in these 
sroups, though not apparently in Holothurians, the ambula- 
cral system has failed to extend itself over the region of the 
right body-cavity. It must be admitted, however, that this is 
also true of Crinoids, in which the terminal plates do not exist. 
The organs hitherto considered may (though this is uncertain) 
have assumed a radial arrangement before the separation from 
the parent stem of any of the recognised classes ; but there are 
a few cases of radial symmetry which must have been arrived 
at independently by the groups in which they occur, This is 
the case in the water-pores (five in number) of Rhizocrinus, 
which seem to form a primitive feature in Crinoids (since 
Antedon passes through such a stage), but are not found in any 
other Echinoderms. Whether four of them were developed 
(simultaneously (one of course being primitive), or in succession 
as in Antedon), is not quite evident. 
A similar case of the acquisition of radial symmetry by indi- 
vidual organs is seen in the form of the stomach in Asterids 
and Ophiurids. In Echinids, Holothurians, and Crinoids the 
alimentary canal is a thin tube winding round the disc without 
any trace of radial arrangement. 
