ECHINODERMATA 



253 



the numerous feet acting in unison may slowly pull the animal 

 along. The spines frequently assist in locomotion, in which 

 case they are long, jointed at the base and operated by special 

 muscles. In some cases the tube feet lack the sucking disc and 

 serve either as sense organs or 

 organs of respiration. 



There is a body cavity in 

 which the digestive and repro- 

 ductive organs are freely sus- 

 pended. The circulatory system 

 is not well developed. Special 

 sense organs of a simple tyJDe 

 sometimes occur. The nervous 

 system is also poorly developed, 

 consisting chiefly of a ring of 

 nerve fibres encircling the mouth 

 and sending a radial nerve into 

 each ray of the body. 



551. Class I. Pelmatozoa. — 

 The crinoids or ''sea lilies" are 

 the only living representatives 

 of this group. The body is at- 

 tached by a stalk-like develop- 

 ment of the aboral pole. In a 

 few forms this is true only in 

 the young stages but for most 

 families the stalked condition is 

 permanent. The skeletal ele- 

 ments are highly developed. The stalk, body and arms are 

 largely composed of calcareous joints and plates. The five rays 

 are usually repeatedly branched. The ambulacral system 

 consists of ciliated tentacles which serve for respiration and to 

 maintain the currents by which food is carried to the mouth. 

 In connection with the latter function, cihated grooves on the 



Fig. 144. 



Antedon, a Crinoid. 

 X 1/2. 



