It is believed also that they secrete bactericidal substances and ferments which bring about the liquefaction of the fibrin and the damaged tissues - histolysis - and thus assist the process of absorption.
The changes have been found to be made up of two sets of processes: histolysis, by which the whole or part of a structure disappears: and histogenesis, or the formation of the new structure.
By histolysis certain parts of the hypodermis are destroyed, while other portions of it develop into the new structures.
Hence the opinion arose that histolysis is a process of phagocytosis.
The periodical histolysis may be partly due to the absence of specific excretory organs and to the accumulation of pigmented excretory substances in the wall of the alimentary canal.