Shellfish morphology and structure

Shellfish's body is soft, symmetrical, and has no sections. It consists of five parts: head, axe foot, visceral sac, mantle and shell. The head has sensory organs such as mouth, eyes and antennae. The axe foot is on the ventral surface of the body and is composed of strong muscles. It is an organ for crawling, digging in sand or swimming. The visceral sac is located on the back of the body and includes internal organs such as the heart, kidneys, stomach, intestines, digestive glands and gonads. The mantle covers the outside of the body and is composed of the inner and outer layers of the epidermis, the connective tissue between them, and a few muscles. The epidermal cells of the mantle secrete shells, and the mantle and shells are the protective organs of shellfish.

The nervous system of shellfish is composed of four pairs of ganglia, brain, foot, side, and viscera, and the nerves connected to it. The cranial ganglion is located on the dorsal side of the esophagus and sends nerves to the head and front of the body; the foot ganglion is located at the front of the foot and sends nerves to the foot; the lateral ganglia is located at the front of the body and sends nerves to the mantle and gills; The ganglion is located at the back of the body and sends nerves to the internal organs. The primitive types of shellfish have a simple nervous system, without significant ganglia. The more evolved types form ganglia, and the more evolved types have ganglia concentrated in the head to form a "brain". The sensory organs mainly include antennae, eyes, balance sac, sniffing device and so on.