And this substance is attached in two places to the membrane and the skin, obliquely; and at the point of attachment the space is narrowed at each side, where the fleshy substance stretches towards the passages that lead outwards through the shell; and here it discharges and admits food and liquid matter, just as it would if one of the passages were a mouth and the other an anal vent; and one of the passages is somewhat wider than the other one.