186 P, HERBERT CARPENTER. 
periheemal canals for the oral ring and the central plexus, 
though Ludwig speaks generally of the circumvisceral 
ccelom as representing the perihemal canals of the Asterids 
(21). 
We have seen that the central plexus which unites the 
oral and aboral rings of the Starfishes does not terminate 
in the latter, but passes on towards the dorsal portion of the 
disc, attaching itself to the under surface of the perisome, 
just outside the madreporite. In the Crinoids, however, the 
case is different. Towards the lower portion of the visceral 
mass the vessels of the central bundle group themselves into 
an inner set, surrounded by a ring of five vessels, which are 
radially disposed. After passing down the central funnel 
between the first radials (figs. 14, 15 R.), and traversing the 
median opening of the rosette (7.), these five vessels expand 
into five large chambers (ch.), which are regularly arranged 
around the central vascular axis. The structure thus formed, 
which is known as the “‘ chambered organ,” is lodged within 
the cavity of the centro-dorsal piece (cd.) of Comatula, and is 
covered in by the rosette (7); but in the stalked Crinoids it is 
situated in some portion of the space enclosed within the 
basals and radials (7, 8). 
This chambered organ is enclosed in a fibrillar envelope 
(x), processes of which extend through all the joints of the 
rays and arms (figs. 9, 11—15, a.) and also into the cirrhi, 
that are borne on the centro-dorsal piece of Comatula (c?). 
Each of these fibrillar cirrhus cords encloses a small vessel 
(ct. v.). Those of the oldest or first-formed cirrhi are derived 
from the vessels of the central axis of the chambered organ, 
while the vessels of the last-formed marginal cirrhi arise 
directly from the five peripheral chambers. These cham- 
bers are not completely closed below, each having a small 
opening in its floor close to the central axis. This opening 
is a relic of the larval stage of existence, when the five 
chambers and the vessels of the central axis were prolonged 
downwards into the stem. In the stalked Crinoids this 
condition is permanent through life. The stem of Penta- 
crinus, Rhizocrinus, or Bathycrinus contains a central vas- 
cular axis enclosed in a downward extension of the fibrillar 
envelope of the chambered organ (7, 8, 20). Like the 
chambered organ this axis consists of five peripheral vessels 
surrounding a core of smaller ones, and from these periphe- 
ral vessels are derived the central vessels within the cirrhi 
borne by the stem. In Pentacrinus these cirrhi are borne 
by special nodal segments which occur at more or less 
regular intervals all down the stem, The five large peri- 
