Fig. 2. Feeding movements of (A) Lymnaea and (B) Aplysia. (A) Schematic diagram of the feeding sequence of Lymnaea in which the three phases of movement (protraction, rasp and swallow) are shown diagrammatically. The muscles of the buccal mass — principally the anterior and posterior jugalis (aj, pj) and radular tensor (rt) — contract, squeezing and rotating the odontophore (OD). This is covered by the many-toothed radula (R), which is rasped across the substratum. Food particles are then collected and passed into the oesophagus (OE). M, mouth. After Benjamin and Elliott (1989). (B) A biting sequence in Aplysia. The food that elicited the movement is not shown. (i) The animal is in the head-up feeding position. (ii) The animal moves its head and lips to centre the food close to the mouth. (iii) The mouth opens, and the radula protracts forward out of the mouth. Note that the radula halves are open during the protraction. (iv-vi) The radula gradually retracts and the mouth closes. Note that the radula halves close. Closure coupled with the retraction pulls the food into the mouth. After Kupfermann (1974).