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. 



Class IV. ECHIIfOIDEA (Sea Urchins). 



Chaeacteeistics. — Spheroidal or heart-shaped Echinodermata, 

 sometimes flattened dor so-v&ntr ally. The calcareous ossicles 

 take the form of definitely-arranged plates usually immovably 

 united iy their edges, and of moveable spines. The number 

 of radii always five in recerd 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 



