shellfish

Shellfish is a type of mollusk. It is an animal with three germ layers, symmetrical on both sides, and a true body cavity. The true body cavity of mollusks is formed by the cleft cavity method, which is the body cavity formed by the mesoderm. But the true body cavity of mollusks is underdeveloped and only exists in the pericardial cavity and gonad cavity. Molluscs vary greatly in morphology, but in structure they can be divided into four parts: head, feet, visceral sac and mantle. The head is located at the front of the body, the feet are located behind the head and on the ventral surface of the body. It is a multi-muscular motor organ protruding from the body wall. The visceral sac is located on the back of the body.

It is an internal organ surrounded by a soft body wall. The mantle is One or a pair of membranes formed by the body wall extending and drooping on the back of the body, the cavity between the mantle and the visceral sac is the mantle cavity. Calcium carbonate is secreted from the mantle to the body surface, forming one or two shells to surround the whole body. A few kinds of shells are surrounded by the body wall or the shells disappear completely. These basic structures have great changes and differences in different classes.

Molluscs have a complete digestive tract, respiratory and circulatory systems, and metanephridium, which is more evolved than the original kidney. There are many kinds of molluscs and they are widely distributed. There are more than 110,000 species and 35,000 fossil species in existence, making it the second largest category in the animal kingdom after arthropods. In particular, some mollusks use the "lungs" to breathe, and their bodies have the ability to regulate water, making molluscs and arthropods the only terrestrial invertebrates suitable for living on the ground.