EOSACEUS, LINCK (COMATULA EOSACEA OE LAMAECK). 
541 
and brachial grooves. The joints or plates of the radial system may be at once distin- 
guished by their being chiefly made up of the peculiar fasciculated (or radial) tissue of 
parallel rods which I have already described, and by their being perforated for the 
lodgment of a sarcodic axis. At first each radial element appears to consist of two parts. 
A stem-joint always commences with an annular spicule, within which the cylinder of 
“radial” tissue seems to arise. An arm-joint begins with a crescentic spicule, and a 
radial plate with an expanded single cribriform film. From the strong contrast which 
these superficial portions present to the tissue which is afterwards developed beneath 
them, I am inclined to refer the outer rings and films, even of the brachial joints and 
radial plates, to the perisomatic system, and to regard the radial system of plates as 
composed essentially of the “radial” tissue alone. The plates and joints of the radial 
system are singularly uniform in their structure and arrangement throughout the whole 
of the crinoidal series. 
They seem to form, as it were, an essential skeleton whose constant general arrange- 
ment stamps the order with its most important and prominent character. In the Pen- 
tacrinoid the radial system of radial- and arm-joints supports the extensions of the radial 
vessels, and the radial vessels with their oesophageal vascular ring clearly arise in con- 
nexion with the disk, on the oral aspect of the animal. The radial plates arise at the 
opposite or apical pole. The first portion of the radial system which appears is the stem. 
When the sarcode-axis of the stem enters the cup, passing through the centro-dorsal 
plate and between the lower edges of the basals, it splits into five threads which enter 
the first radial plates, and after a somewhat singular distribution in the walls of the cup, 
which is not apparent till a later stage, they follow out the growing arms, the arm-joints 
being moulded round them as they extend. The perivisceral sac lies in the cleft formed 
by the five radial branches of the stem. The plates of the perisomatic system commence 
as simple cribriform films imbedded in the outer layer of the perisom, and thicken by a 
repetition inwards of the same diffuse areolar tissue. They are essentially variable in 
number and in arrangement ; most of the minor structural modifications throughout the 
group depend upon the multiplication or suppression of plates of this series. Even in 
the same species they are by no means constant. In Antedon rosaceus the perisom of 
the disk is usually naked, but specimens from certain localities have well-defined groups 
of perisomatic interradial plates developed in the angles between the radial axillaries, 
and in some individuals rows of similar plates are imbedded along the margins of the 
radial grooves in the perisom of the disk. The entire body of the Pentacrinoid is, at 
first, while yet included within the pseudembryo and during its earliest fixed stage, 
surrounded and enclosed by plates of the perisomatic system alone, and it is quite con- 
ceiveable that plates belonging to this system may expand and multiply so as to form a 
tessellated external skeleton to the mature animal, the radial system being entirely absent, 
or represented only in the most rudimentary form. I believe that all the modifications 
of the skeleton Avhich characterize the principal divisions of the Echinoderm subkingdom 
will be found to depend mainly upon the relative development or suppression of the 
radial and perisomatic systems of plates. 
