There is strong evidence for supposing that the Mollusca are derived from a bilaterally symmetrical animal, having a median digestive tract and lateral excretory organs. This animal was further characterised by the physiological character of its dorsal integument, which had the property of secreting a peculiar substance—conchiolin. This, though in itself capable of affording some protection to the animal, becomes impregnated with carbonate of lime, thus providing the animal with a hard coating sufficient to resist the attacks of all but the most powerful of its enemies, while at the same time it forms a strong external skeleton to which the muscles of the body can be attached. This conchiolin, however, unlike chitin, does not seem to lend itself to the formation of jointed appendages or metameric segmentation, with which we are so familiar in another group, the Arthropoda.