Cephalopod horny crust

Among the existing cephalopods, only the Nautilus has a shell. The shell is symmetrical on both sides, and the plane is hovering. There are many transversal septa in the shell to divide the shell into many small compartments. Weigh living room. This series of small chambers are formed as the animals continue to grow and the rear end of the body continues to secrete partitions, so that the shells continue to grow. The partitions are in the shape of a half moon, concave in the front and protruding in the back. The connection between the two ends of the plate and the shell is called suture. The center of the partition has a small protrusion protruding backward, and the center has a hole. During life, there are living body tissues connected between the front and back protrusions to form a siphuncle that runs through all the shell chambers. The body tube is inflated or liquid to control the vertical movement of the body in the ocean. The extinct fossil nautilus, the shape of the shell changes greatly, the shell may be straight tube, or the rear end of the straight tube is curled or spiraled, and the suture of the shell plate is simple or complex, such as in the Cambrian strata. The sutures of the Nautilus fossils of, are simple and mostly linear. The sutures of the Ammonoidea shells appearing in the Silurian strata are complex, which reflects the level of their evolution.

In the existing cephalopods, except for the outer shell of Nautilus, the other species have inner shells or disappeared shells, and the shape of the inner shells is not exactly the same. It is generally believed that this is the development of the extinct Belemnites in different directions, resulting in the formation of different types of inner shells or the disappearance of the shells. The shells of the archipelago are cone-shaped, one of which forms the existing deep-sea Spirula through the rotation of the shell; the partition of one shell disappears, leaving only the back wall of the stratum corneum as the attachment point for the muscles And the support of the body, that is, the shell of the existing calamari (Loligo); a partition that retains the shell, but the shell degenerates into a flat boat shape, only as a support for the body, such as the shell of a squid; The support shell completely degenerates and disappears, such as the octopus, the trend of this change is to reduce the weight of the shell to facilitate body movement. As the shell gradually degenerates, cartilage formed by the mesoderm appears in the body to increase the body's support and protection.