Molluscs: Secondary loss of the shell
One of these group of molluscs took the same path as the sea slugs and disposed of its shell entirely (octopuses and squids) whereas relict of the ancestral shell persist as the cuttlebone found in the cuttlefishe. One species of octopus (Argonauta) secretes a paper-thin replica of the nautilus shell, the chambers of which are used to lays its eggs.
ARGONAUTA ARGO FEMALE WITH EGGCASE
Both squids and octopuses have reduced the number of tentacles (10 and 8 respectively), but squids have become more mobile with the development of undulating lateral fins. The brains and eyes of these animals is the most advanced of any invertebrate, eyes greater than 400 mm in size have been recorded for squid. Squids, in particular can reach immense sizes with one individual 21 m long (found in New Zealand in 1933).