(am-fee-pro-ra). Pennate diatoms are usually regarded as boat shaped, although some of the boats have very odd shapes. They can usually glide. This particular form is very common in sediments - the form apparently allowing the cell to penetrate through the pores of the sediments. Although enclosed in a siliceous shell, the shell of these rather delicate diatoms is flexible. The plastids contain chlorophylls a and c which gives the yellowy-brown colour. Phase contrast.