274 PHYLUM ECHINODERMA 
**centro-dorsal” ossicle, bearing the cirri; this conceals the coalesced 
‘*basals” of the larva; above this are three tiers of ‘‘ radials,” whence 
spring the ‘‘ brachials” of the arms. 
The oral disc, turned upwards,. is supported by plates. Here the 
anus also is situated. The arms usually branch in dichotomous fashion, 
and thus ten, twenty, or more may arise from the original five. But the 
growing point continues to fork dichotomously, like the leaf of many 
ferns, and as each altcrnate fork remains short, a double series of lateral 
‘*pinnules” results. The arms are supported by calcareous plates. The 
stalk usually consists of numerous joints, especially in extinct forms, in 
some of which it measured over fifty feet in length. Except in Holopus, 
Hyocrinus, and in the stalked stage of Antedon, the stalk bears lateral 
cirri. 
The nervous system consists (a) of a circumoral ring with ambulacral 
nerves, and (4) of axial coelomic nerves up the ossicles on the opposite 
side of each arm and connected with » peculiar ‘‘ chambered organ” 
in the interior of the centro-dorsal plate. 
Apart from the superficial epithelium, there are no sensory structures. 
The ciliated food canal descends from the mouth into the cup, and 
curves up again to the anus, which is on a papilla. The last part of 
the gut is expanded to form an anal tube, which during life is in con- 
stant movement, and has apparently a respiratory function. From the 
cup, where the body cavity is in great part filled with connective tissue 
and organs, four coelomic canals extend into each of the arms. They 
communicate at the apices of the arms and pinnules, and currents pass 
up one and down the other. 
The blood-vascular system consists of a circumoral ring, which is 
connected with a radial vessel under each ambulacral nerve, and with a 
circum-cesophageal plexus. 
The water-vascular system consists as usual of a circumoral ring and 
radial vessels, but in several respects it shows remarkable modification. 
The madreporite of other forms is represented by fine pores which open 
from the surface of the calyx directly into the body cavity, and which 
may be very numerous ; there are said to be 1500 in Aztedon rosacea. 
By these pores water enters the body cavity, and from it enters the 
numerous stone canals which hang from the ring freely in the body 
cavity, and open into it near the pore canals. There are no Polian 
vesicles or ampullz, the tube-feet are small, are arranged in groups of 
three, and are connected by delicate canals with the radial vessels. 
Certain of them form tentacles around the mouth, and these are supplied 
by canals coming off directly from the ring canal. 
The sexes are separate. The reproductive organs extend as tubular 
strands from the disc along the arms, but are rarely functional except 
in the pzznules, from each of which the elements burst out by one duct 
in females, by one or two fine canals in males. 
The oval ciliated larva of Amtedon, the only one known, is less 
peculiar than that of other Echinoderms, 
There are about 400 living species in twelve genera, but about 1500 
species in 200 genera are known from the rocks. The class is obviously 
decadent. It is represented in the Cambrian, and attained its maximum 
development in Silurian, Devonian, and Carboniferous times. 
