▁in ▁its ▁shape , ▁with ▁the ▁or al ▁( lower ) ▁surface ▁being ▁flat ▁and ▁the ▁ab oral ▁( upper ) ▁surface ▁marked ly ▁de pressed ▁in ▁the ▁centre . ▁The ▁t uber cles ▁are ▁numerous ▁and ▁even ▁in ▁size , ▁and ▁the ▁sp ines ▁are ▁fine ▁and ▁short , ▁their ▁diameter ▁being ▁about ▁a ▁quarter ▁of ▁their ▁length . ▁The ▁inter - amb ul ac ral ▁pl ates ▁are ▁broad ▁and ▁the ▁p ore ▁pairs ▁are ▁arranged ▁in ▁slightly ▁cur ved ▁groups ▁of ▁six ▁or ▁seven ▁pairs . ▁ ▁E col ogy ▁As ▁is ▁the ▁case ▁with ▁most ▁other ▁sea ▁ur ch ins , ▁the ▁sex es ▁are ▁separate ▁in ▁this ▁species , ▁and ▁adult s ▁liber ate ▁eggs ▁and ▁s perm ▁into ▁the ▁water ▁column . ▁After ▁fert il isation , ▁the ▁e chin op l ute us ▁lar va e ▁spend ▁several ▁months ▁sw imming ▁and ▁dr ifting ▁with ▁the ▁pl ank ton . ▁Met am orph osis ▁is ▁stim ulated ▁by ▁the ▁detection ▁by ▁the ▁lar va e ▁of ▁suitable ▁locations ▁to ▁sett le ▁on ▁the ▁se ab ed , ▁c ues ▁being ▁provided ▁by ▁the ▁presence ▁on ▁the ▁substr ate ▁of ▁films ▁of ▁micro - al ga e ▁or ▁of ▁micro bes . ▁In ▁the ▁absence ▁of ▁a ▁suitable ▁habitat , ▁the ▁lar va e ▁can ▁continue ▁sw imming , ▁but ▁met am orph osis ▁will ▁eventually ▁happen ▁even ▁in ▁the ▁absence ▁of ▁these ▁c ues . ▁ ▁Us es ▁P . ▁de press us ▁is ▁used ▁for