Cephalopod system

The body of cephalopods is also composed of four parts: head, feet, visceral sac, and mantle, but in terms of system structure, it can be divided into three parts: head, trunk and funnel. The primitive species had a well-developed hovering shell outside, such as Nautilus, which is the only genus that exists. Other living species have symmetrical sides and underdeveloped shells to form inner shells, such as cuttlefish (Sepia), or their shells completely degenerate and disappear, such as octopus (Octopus).

Compared with the original molluscs, the body structure of cephalopods may change from crawling to swimming, so that the dorsal-ventral axis of the body becomes the front and rear axis of the body. As a result, the head is located at the front of the body, and the visceral sac is located at the back of the body. The mantle that was originally located on the back of the original mollusk has a muscle layer and extends to surround the visceral sac to form the trunk.

The mantle cavity is moved from the original back to the body. Ventral. The wide feet originally located on the ventral surface of the original mollusk also moved forward with the change of the body axis. A part of the foot constitutes the wrist (or tentacles) of the head, and a part constitutes the funnel on the ventral front of the torso. After the free abdominal edge of the mantle is connected, it becomes the water outlet of the mantle cavity. The role of cilia is important in other classes of mollusks.

The cilia on the mantle and comb gills cause water to flow through the body to complete the functions of feeding, breathing, excretion, and reproduction. But the function of the cilia of cephalopods has been replaced by the developed muscles on the mantle. The contraction of the muscle causes water to flow in from the mouth of the outer jacket, and then eject from the mouth of the funnel. The role of cilia in the digestive tract is also replaced by muscles, which is closely related to the swimming and predatory habits of cephalopods.