The siphons are outgrowths of the mantle while the sheath is a continuation of the periostracum .
They are streaked with brown and purple and are housed in a transparent , gelatinous sheath composed of protein and chitin .
The siphons are outgrowths of the mantle while the sheath is a sheath of the periostracum .
They are streaked with brown and purple and are housed in a transparent , gelatinous sheath composed of protein and chitin .