524 
PROFESSOR W. THOMSON ON THE EMBRYOGENY OF ANTEDON 
We have thus the rudiments of the “ pentacrinoid stage” of the Antedon clearly 
defined and rapidly advancing in development within the body of the pseudembryo, 
while the latter still retains in perfection its independent form and its special organs of 
locomotion and of assimilation. 
I have found it utterly impossible at this stage to trace the formation of the viscera 
of the young pentacrinoid, on account of the close calcareous network in which the 
nascent organs are enveloped. From its colour and position, however, there can be no 
doubt that the mass occupying the base of the cup represents the origin of the stomach 
with its granular hepatic folds, while the upper more transparent sarcode-hemisphere 
indicates the nascent tissues of the vault, and at a subsequent stage originates the ambu- 
lacral ring with its radial branches and the tissues of the young arms. The two rows of 
plates, enclosing the viscera and forming the cup at this early period, represent the basal 
and the oral series of plates, which are remarkably suppressed and modified during the 
subsequent development of the crinoid. The jointed calcareous rod is the stem of the 
Pentacrinoid, and the circular calcareous plate afterwards supports the round fleshy 
disk by which the base of the stem adheres to its point of attachment. From six to 
twenty-four hours later the pseudembryo becomes more sluggish in its movements, and 
begins to lose its characteristic contour. The anterior extremity becomes somewhat 
flattened, and then slightly depressed in the centre. The stem of the included crinoid 
lengthens, and the sarcode of the body of the pseudembryo contracts towards it. The 
pseudostome and pseudoproct become obscure and are shortly obliterated, the sarcode 
forming a thick, smooth, uniform layer over the stem and over its terminal disk. The 
two posterior ciliated bands disappear, the anterior bands remaining entire a little 
longer, and still subserving the locomotion of the pseudembryo. The anterior bands 
then likewise gradually disappear, the pseudembryo sinking in the water and resting 
upon a sea-weed or a stone, to which it becomes finally adherent. 
At this stage the pseudembryo is irregularly oval and in form slightly contracted 
posteriorly, expanded and gibbous anteriorly, the anterior extremity flattened or slightly 
cupped. The posterior extremity expands into a small rounded disk (Plate XXV. fig. 1). 
Slightly compressed and examined by transmitted light, the Pentacrinoid larva has but 
little altered from the description given above; the joints of the stem are somewhat 
lengthened, and the cup is rather more open by the growth and slight separation of the 
upper portions of the plates of the upper tier. The whole of the pentacrinoid is 
entirely invested by a thick layer of transparent sarcode, which is merely the substance 
of the body of the larva which has contracted uniformly over the body and stem of the 
crinoid, its surface retaining, with the exception of the absence of the bands of cilia, the 
same character as the surface of the pseudembryo, with the same pyriform oil-cells 
arranged in the same way, and leaving the same interstices of nearly transparent deli- 
cately vacuolated sarcode. The head of the crinoid now becomes more regularly pyri- 
form, and the stem rapidly lengthens. The posterior disk becomes firmly and perma- 
nently fixed to its point of attachment. The wide anterior extremity now shows a 
