240 ZOOLOGY 
system dilate into chambers which give off vessels to the cirrhi. 
Above they communicate with a plexus of vessels around the 
oesophagus, this plexus communicates with the distal portion 
of some of the stone canals. 
The chief nervous system is situated dorsally ; it consists of 
a mass of nervous matter lying within the circle of basal 
ossicles, and giving off a large nerve to the stalk, which 
supplies branches to all the cirrhi, and five radial nerves, each 
of which divides into two, and the resulting nerves supply 
each arm and govern their movements. This system is con- 
tinued into the pinnules; it is probably connected here and 
there with the ambulacral system of nerves, whose function 
seems to be mainly sensory. This dorsal or anti-ambulacral 
system may be derived from concentrations of a subepidermal 
nervous system, such as exists in Asterids, which have sunk 
into the body. 
Crinoids are attacked by an order of highly-modified 
Chaetopods, termed Myzostomidae. These occur only on the 
Crinoidea, and live parasitically either on the disk or arms ; 
their presence often causes local abnormalities of growth, pro- 
ducing swellings sometimes termed galls. The order includes 
two genera, Myzostoma and Stelechopus. Extinct Crinoids seem 
to have suffered from the same parasite. 
Crass IV. ECHINOIDEA (Sea Urchins). 
CHARACTERISTICS.—Spheroidal o7 heart-shaped Echinodermata, 
sometimes flattened dorso-ventrally. The calcareous ossicles 
take the form of definitely-arranged plates usually immovably 
united by their edges, and of moveable spines. The number 
of radii always five in recent forms. Mouth and anus present. 
A ciliated ectoderm covers the body of the Echinoids, 
beneath this is a nerve plexus. The calcareous plates which 
constitute the shell of the animal are developed in the con- 
nective tissue of the integument. The apical series of plates 
consists of a dorso-central piece surrounded by ten plates ; five 
of them, the radials or ocular plates, bear sense organs, the 
alternating five, interradial in position, are pierced by the genital 
pores. The ambulacral plates abut against the radials, and it is 
