▁shell . ▁The ▁Ox yn o ace a ▁contains ▁three ▁sh elled ▁families , ▁and ▁all ▁feed ▁sole ly ▁ ▁on ▁alg ae ▁of ▁the ▁genus ▁Ca uler pa . ▁None ▁of ▁these ▁organ isms ▁benefit ▁from ▁the ▁photos yn thesis ▁of ▁the ▁ing ested ▁ch lor op last s , ▁but ▁there ▁is ▁some ▁suggestion ▁that ▁the ▁ch lor op last s ▁may ▁have ▁been ▁retained ▁in ▁order ▁to ▁perform ▁a ▁cam ou fl aging ▁function . ▁The ▁shell s ▁of ▁the ▁Vol vat ell idae ▁and ▁Ox yn o idae ▁somewhat ▁res emble ▁those ▁of ▁the ▁Ce phal asp id ▁b ubble ▁sn ails . ▁The ▁Juli idae ▁are ▁extraordinary ▁in ▁that ▁they ▁are ▁sh elled , ▁b ival ved , ▁g ast rop ods . ▁They ▁have ▁a ▁shell ▁in ▁two ▁pieces ▁which ▁res emble ▁the ▁val ves ▁of ▁a ▁minute ▁cl am . ▁Living ▁members ▁of ▁this ▁family ▁have ▁been ▁known ▁since ▁ 1 9 5 9 , ▁and ▁had ▁previously ▁only ▁been ▁known ▁to ▁science ▁as ▁foss ils ▁( which ▁had ▁been ▁interpreted ▁as ▁b ival ves ). ▁ ▁Pla k ob ran cho idea ▁▁ ▁The ▁majority ▁of ▁sac og loss ans ▁are ▁shell - less , ▁consequ ently ▁the ▁pla k ob ran cho idea ▁are ▁commonly ▁described ▁using ▁the ▁vern ac ular ▁term ▁" se a ▁sl ugs ", ▁which ▁can ▁lead ▁to ▁their ▁confusion ▁with ▁the ▁only ▁very ▁dist antly ▁related ▁n ud ib ranch s . ▁ ▁However , ▁the ▁pla k ob