Some notes on the fine structure of Arcella shells (and an answer to a question nobody asked)
Amoebae in the family Arcellidae make their shells from a self-produced organic substance, without incorporating any other building materials (diatoms, hard scales, bits of quartz etc.). Although described in older sources as “chitinous,” and in newer ones as “keratin-like”, the precise composition of this substance is still unknown. The stuff is not laid down as…