The histochemical analysis results were as follow. In the inner mantle fold, the epithelial cells were strongly NOS-positive, the haemocytes and nerve fibers in the connective tissue were NO S-positive, the muscle fibers were weakly NOS-positive. In the mantle tentacle, many round cells and wavelike nerve fibers presented strongly NOS-positivity. In the margin of mantle, there were many strongly NOS positive nerve cells and positive nerve fibers; abundant strongly NOS-positive round cells were detected near the pallial retractor muscle and inner epithelia of blood vessel. In the center of mantle, the distribution of NOS-positive cells were very different in different parts, abundant round NOS-positive cells distributed in gathering or scattering in the epithelial tissue, there were many round NOS-positive cells near the inner epithelia of blood vessel too.

The immunohistochemical analysis showed that the haemocytes and epithelial cells of the inner mantle fold and margin of mantle presented weakly positive reaction of neuronal nitric oxide synthase (nNOS) and endothelial nitric oxide synthase (eNOS), and presented positive reaction of inducible nitric oxide synthase (iNO S). The nerve cells, nerve fibers and inner epithelial cells of blood vessel presented nNO S, eNOS and iNOS negative reaction.

The biochemical test showed that both NOS and NO were detected in each parts of mantle. The enzyme activity level of total nitric oxide synthase (?\TOS) and contents of NO in center of mantle were the highest in all parts of the mantle, the ?\TOS activity and contents of NO in mantle margin were higher than the mantle tentacle or the inner mantle fold, and the lowest part was the inner mantle fold. In both the central mantle and inner mantle fold, the iNOS activity was higher than cNOS activity; in the margin of mantle, the iNOS activity level was close to the cNOS activity level; in the mantle tentacle, the iNOS activity level was much lower than the cNOS activity level.

The results suggested that the NO-NOS system in mantle of C. farreri may play an important role in its immunological defense and immunoregulation, and the mantle may be the produce center of NOS.