33 



of chitin are supported by a tough sheet of connective tissue , known as the 

 subradular membrane. At the ventral end of the radular sac the tooth bearing 

 portion of this membrane is reflected downward over the lower surface of the 

 odontophore between it and the tongue , while the toothless portion of the mem- 

 brane is reflected backward over the sides and top of the odontophore. The 

 teeth (Text figure 12) are arranged in seven longitudinal and an indefinite number 

 of transverse rows. Each transverse row consists of a central tooth flanked by 

 three pairs of lateral teeth and has , at each end , a flat oval plate of chitin. 

 The central tooth has a strong middle point and a pair of lesser lateral points. 

 The tooth at either side of the center consists of a large inner and a smaller 

 outer point. The teeth of the second and third rows from the center are strong, 

 long , curved spikes. New teeth are constantly produced at the base of the radula 

 and gradually push outward until they reach the mouth of the sac where they 

 are drawn downward and then back- 

 ward over the end of the odonto- 

 phore. While in the radular sac, the 

 points of the teeth project toward 

 its base (backward) but as they are 

 drawn over the end of the odonto- 

 phore the teeth point first upward 

 and then forward so that they rasp 

 food only as the radula is drawn into its sac. 



Two pairs of muscles, the radular retractors and protractors, draw the radula 

 in and out over the apposed ends of the two radular cartilages which support it. 

 These cartilages lie at the side of the radular sac and are slightly curved upward 

 at each end. The dorsal ends of the rod-shaped cartilages are fastened to the 

 lower part of the dorsal end of the upper jaw by means of muscle fibres which 

 radiate backward, inward and forward from the jaw to the cartilage. The upper, 

 i. e. the concave , side of each cartilage is covered , except at the end , by a trans- 

 verse muscle which is several times as thick as the cartilage. The fibres of this 

 muscle are sufficiently curved to be concentric with the cartilage and arise from , 

 and are attached to , a fascia which extends from one end of the fibres down- 

 ward under the cartilage and up on the opposite side to the other end of the 

 same fibres. The only action which I can assign to this peculiar muscle is that 

 the thickening of its fibres during contraction , presses downward the ends of the 

 raduar cartilage, straightens it and thereby forces the radular forward. The 

 radular cartilages are connected by a sheet of muscle which stretches beneath 

 the radular retractors. A similar but thicker and narrower sheet of muscle 



Fig. 12. 



