166 HUGH WATSON. 



A longitudinal muscle runs along the upper edge of the 

 support on each side towards its anterior end (PI. XVI, fig. 

 90; PI. XVIII, figs. 116-118), and probably serves to bend 

 up the front of the support, as shown in PI. XVII, fig. 108, 

 and may therefore be termed the flexor muscle of the odonto- 

 phoral support. In Apera gibbon si there is also a very 

 thin layer of longitudinal muscle-fibres covering the ventral 

 surface of the support, and possibly the flexor muscles 

 might be regarded as a special thickening of this layer. 



The most important muscles in the odontophore are the 

 powerful retractors, which arise from the odontophoral 

 support and are inserted in the radula-sac. These may be 

 divided into lateral retractors, median retractors, and terminal 

 retractors. The numerous lateral retractors are attached in 

 front to the radula-sac, chiefly around its anterior end, and 

 pass obliquely backwards and outwards on each side, curving 

 over the edges of the support and becoming attached to its 

 outer sides (PI. XVII, figs. 92, 93,96, 101, 106, 109, 111, 113, 

 and PL XVIII, figs. 116-121). The median retractors are a 

 pair of specially thick muscles, which are attached for some 

 distance to the floor of the radula-sac towards its anterior 

 end, and pass backwards to the extreme hind end of the 

 support (PL XVII, figs. 92, 106, and PL XVIII, figs. 118-122). 

 They belong to the same series as the lateral retractors, of 

 which they might be regarded as forming the innermost pair. 

 No muscles are attached to the posterior part of the radula- 

 sac, excepting at its hind end, where there is another pair of 

 retractors, which I am calling the terminal retractors. These 

 are not so thick as the median retractors, and pass backwards 

 above them to the hind end of the support (PL XVII, fig. 92, 

 and PL XVIII, figs. 121, 122). All these retractor muscles 

 have the same function — that of pulling back the radula. 

 In A. gibbonsi some of the more posterior strands of 

 muscle are attached to the inner surface of the support, close 

 to its hind end, while others are not attached to the support 

 at all, but to the hind end of the sheath of the odontophore 

 opposite to the insertion of the extrinsic buccal retractors. 



