540 
PEOFESSOE W. THOMSON ON THE EMBEYOGENT OF ANTEDON 
single layer of cribriform calcified tissue. The plates shortly begin to thicken ; but 
their mode of growth at once distinguishes them as fundamentally different in structure 
from the basals and orals. Processes are sent inwards from the inner surface of the 
superficial film as before ; but the added tissue is longitudinal and fasciculated, resem- 
bling precisely in structure and mode of growth the inner cylinder of the joints of the 
stem ; and, as in the case of the stem, tubular perforations are formed in it for the 
passage of the sarcode-cords, which subsequently extend in like channels through the 
joints of the arms and pinnules. The second radial joints and the radial axiilaries rapidly 
succeed the first radials, and are developed nearly in the same way. They first appear 
as horseshoe-shaped spicula, or imperfect rings, which have the same relation to the 
joints which the stem-rings have to their included cylinders. The spicula soon become 
filled up with lengthening fasciculated tissue ; the joints at this period are slightly grooved 
longitudinally on their upper surfaces to accommodate the radial vessels. 
The anal plate, the interradial plates, and the plates and spicula of the perisom. — Upon 
the appearance of the second and third radial joints, the perisom between and somewhat 
above two of the first radials rises into a rounded papilla, towards which a csecal process 
of the digestive cavity is directed. On the outer side of this papilla a branching spicule 
appears which rapidly extends into a round plate. This, the anal plate, grows, and 
afterwards thickens precisely on the model of the basal and oral plates ; it contains none 
of the fasciculated tissue proper to the radial system. The basal and oral plates, the 
first and second radials, the radial axiilaries, and the anal plate seem to complete the 
series of essential parts entering into the cup of the pentacrinoid. In one or two cases 
however, I have observed about the time of the first appearance of the anal plate, a 
series of five minute rounded plates developed interradially between the lower edges of 
the oral plates and the upper edges of the basals. These interradial plates sometimes 
remain permanent in the mature Antedon rosaceus, and they appear to be constantly 
present in some species, as for instance in another and a rarer British form, Antedon 
Milleri (Muller). They usually occur, finally, in groups of three or five. They are irre- 
gular in form, and they resemble the anal plate in structure and mode of growth. 
Simple and key-like spicula and small round cribriform plates are imbedded irregularly 
in the perisom of the arms, often almost covering the second and third radial joints with 
a dermal calcified layer, but never overlying the basal or oral plates of the body. 
General remarks on the Skeleton. — The skeleton of the pentacrinoid is composed of 
two systems of plates, which I shall term respectively the radial and the perisomatic 
system, thoroughly distinct in their structure and mode of growth. The radial system 
consists of the joints of the stem, the centro-dorsal plate, the radial plates, and the joints 
of the arms (and subsequently of the pinnules). The perisomatic system includes the 
basal and oral plates, the anal plate, the interradial plates, and any other plates or 
spicula which may be developed in the perisom of the cup or disk. In the recent Pen - 
tacrini, and in certain species of Antedon, the disk is paved or studded with plates 
belonging to the perisomatic system, and a double series of like plates fringe the radial 
