' s ▁body ▁into ▁the ▁empty ▁shell ▁ch amb ers . ▁Through ▁a ▁hyper os m otic ▁active ▁transport ▁process , ▁the ▁am mon ite ▁em pt ied ▁water ▁out ▁of ▁these ▁shell ▁ch amb ers . ▁This ▁enabled ▁it ▁to ▁control ▁the ▁bu oy ancy ▁of ▁the ▁shell ▁and ▁thereby ▁rise ▁or ▁descend ▁in ▁the ▁water ▁column . ▁ ▁A ▁primary ▁difference ▁between ▁am mon ites ▁and ▁na util oid s ▁is ▁the ▁si ph un cle ▁of ▁am mon ites ▁( except ing ▁C ly men i ina ) ▁runs ▁along ▁the ▁vent ral ▁peri ph ery ▁of ▁the ▁sept a ▁and ▁camera e ▁( i . e ., ▁the ▁inner ▁surface ▁of ▁the ▁outer ▁axis ▁of ▁the ▁shell ), ▁while ▁the ▁si ph un cle ▁of ▁na util oid s ▁runs ▁more ▁or ▁less ▁through ▁the ▁center ▁of ▁the ▁sept a ▁and ▁camera e . ▁ ▁Sex ual ▁dim orphism ▁▁ ▁One ▁feature ▁found ▁in ▁shell s ▁of ▁the ▁modern ▁N aut ilus ▁is ▁the ▁variation ▁in ▁the ▁shape ▁and ▁size ▁of ▁the ▁shell ▁according ▁to ▁the ▁sex ▁of ▁the ▁animal , ▁the ▁shell ▁of ▁the ▁male ▁being ▁slightly ▁smaller ▁and ▁wider ▁than ▁that ▁of ▁the ▁female . ▁This ▁sexual ▁dim orphism ▁is ▁thought ▁to ▁be ▁an ▁explanation ▁for ▁the ▁variation ▁in ▁size ▁of ▁certain ▁am mon ite ▁shell s ▁of ▁the ▁same ▁species , ▁the ▁larger ▁shell ▁( the ▁macro con ch ) ▁being ▁female , ▁and ▁the ▁smaller ▁shell ▁( the ▁micro con ch ) ▁being ▁male .