There is a special marginal nerve running round the edge of the mantle, but the connexion of this with the rest of the nervous system is not clear; probably it is merely another concentration of the diffused sub-ectodermal nervous fibrils.
The lophophore is supplied by yet a third nerve, the under arm-nerve, which is less clearly defined than the others, and resembles a moderate aggregation of the nerve fibrils, which seem everywhere to underlie the ectoderm, and which in a few cases are gathered up into nerves.
Then they develop .definite fibrils which differentiate into fibrous laminae forming a zone which shuts off the abscess from the healthy tissue and so prevents the further invasion and injurious effects of the microorganism.
It is from these cells that the fine fibrillar substance is formed, and from this stage onwards - eight to fifteen days - there is a steady increase in the new fibrils, giving more density to the new tissue.
As their fibrils become more developed they gradually form fibrous laminae which are laid down first in the deeper part of the wound.