Cephalopod Morphology

The main features of this class are: the body is divided into three parts: head, neck, and trunk; there are heads and feet, and there are two forms of wrist and tentacles. There is a tubular funnel on the ventral surface of the base of the foot to drain the water in the coat cavity. Eight wrists have 8 long and curlable wrists, with suction cups at the ends of the wrists. 

Squid and Squid have 8 wrists and 2 tentacles, with a suction cup on the tip, and a horny ring inside the suction cup with teeth or hooks. Nautilus has about 90 tentacles without suction cups. The outer shell membrane of cephalopods mostly loses the hard shell and becomes muscular and extremely thick. The mantle is connected to the visceral mass on the back and sides, while the ventral surface is separated to form a mantle cavity, which communicates with the outside through a wide opening under the neck. Except for the Nautilus, the shells of all existing species are extremely degraded, with a straight inner shell (except for Spirula). 

Architeuthis dux is more than 20 meters (66 feet) long including the wrist, and is the largest invertebrate; the smallest is the microfin cuttlefish (Idiosepius), which is less than 2.5 cm (1 inch) in length. Octopus Including the wrist, it is generally no more than 30 cm (12 inches). It is rare that it is more than 1 meter (39 inches), but it has been recorded that the wrist extension is 9 meters (30 feet). Fossil ammonites (Pachydiscus seppenradensis, Cretaceous) are 205 cm (6 feet 8 inches) in diameter and are the largest shell mollusks.