ECHINOIDEA. 



2 33 



nective tissue, there is a network of nerve fibres, and some 

 ganglion cells. Internally, there is another thin layer of 

 connective tissue, and a ciliated epithelium lining the body 

 cavity. The skeleton grows by the formation of new plates 

 around the apical disc, and also by the individual increase 

 of each. In a few forms the shell retains some plasticity. 



ma 



am 



ma, 



am 



Fig. 102. — Diagram of sea-urchin {Echinus). — After Huxley, 

 slightly modified. 



hi., Mouth ; £■. , gut cut through, and with coils omitted ; a., anus ; 

 ma., madreporite ]st., stone canal ; e.c, circular canal ; P., one of 

 the Polian vesicles; r.v. , radial vessel; am., ampulla of tube- 

 foot ; tf., tube-foot ending in sucker ; n., radial nerve given off 

 from nerve-ring ; al., alveolus, one of the parts of Aristotle's 

 lantern, at the left the alveolus is removed to show one of the 

 strong teeth (/.) ',/., falces, to which the retractor muscles (;««.) 

 of the lantern are attached ; sp., spines on surface of test ; pe., 

 pedicellaria ; /., fleshy lobes or lips ; oc, one of ocular plates. 



The nervous system consists of a ring around the mouth, 

 of radial branches running up each ambulacral area, and of 

 the superficial network. Tube-feet, sphseridia, pedicellariae, 

 and spines are all under nervous control, and each radial 



