Morphological, physiological and ecological evidence is used to show that terrestrial gastropods and isopods, although both can be considered as "primary consumers", deal quite differently with the vegetabilic matter they use as food. Gastropods are both efficient digesters and assimilators whereas isopods are efficient digesters but usually inefficient assimilators. This combination may require the isopods to turn to coprophagy as a means of recovering some of the products of digestion that they had been unable to assimilate during the first passage of food through the gut. Furthermore, the faeces of isopods may contain dissolved nutrients and digestive enzymes that could be distributed through various routes of the food web of which these animals are parts.