194 MORPHOLOGY OF INWERTEBRATE TYPES 



pores for the five pairs of oral suckers. The first and second 

 adambulacral oral plates are turned over and fused with the 

 third and fourth plates. The edge of the peristome, turned 

 towards the inside of the test is thickened and presents in each 

 radius a pair of calcareous processes or auricles for the attach- 

 ment of the retractors of the Aristotle's lantern. 



The spines of the aboral surface are more or less sharp-pointed 

 and attain their greatest length in the equatorial region. To- 

 ward the peristome they begin to change their shape, some 

 become blunt at the end, others widen at the end so that the 

 flat end of the spine appears wider than the base. All spines 

 are fluted and all have a hemispherical socket at their base for 

 the articulation with the hemispherical knob of the tubercles. 

 The spine is held in position by a muscular sheath attached to 

 the base of the spine and the periphery of the tubercle. This 

 sheath is composed of a double set of muscular fibres. The set 

 of external fibres is used for locomotion. The set of inner fibres 

 is used for the fixation of the spine in a given direction. The 

 relaxation of the fibres of the sheath brings the spine in its 

 normal upright position. 



The ambulacral tubes are of three kinds. The five pairs sur- 

 rounding the lip are short, heavy and developed as oral suckers 

 or papilla. The five clusters of tubes surrounding the peristome 

 as well as the tubes of the remaining part of the oral surface are 

 real ambulacral feet with a sucker at the end. In life they are 

 thin and long and may be extended beyond the spines. The 

 tubes of the aboral surface have no suckers and appear as 

 tentacles. 



The pedicellarice have a long stem and a head with three jaws. 

 One type of pedicellariae is found on the peristomial membrane 

 and on the test between the spines. The stem is thick and its 

 calcareous axis extends to the head. The head is large and the 

 jaws are wide. The second type is found only on the test. The 

 stem is thin, the calcareous axis does not reach to the head. 

 The head is small and the jaws are narrow. 



