298 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



the two cartilages together, closing the gutter and 

 preventing the radula from slipping up over the sides, 

 whilst the contraction of the median muscle will pull 

 back the radular sheath and even the pharynx. 



There still remains a large number of retractor 

 muscles attached to the dorsal portion of the radula. Of 

 these, two on each side arise from the anterior ends of 

 the lateral odontophoral bands (or the posterior ends of 

 the odontophoral cartilages), and are inserted laterally 

 to the radular sheath (fig. 16, l.r.r.' , l.r.r."). The others 

 all arise on the floor of the proboscis wall some distance 

 posteriorly. They comprise (1) the median muscle 

 lying under the centre dorsal muscle (fig. 16, c.d.m. inf.), 

 (2) the paired muscles (fig. 16, rJ, r."), and (3) the 

 paired muscles (fig. 16, V. r.). 



Altogether, there are four or five pairs of muscles 

 running together in this way, and all are attached 

 laterally and ventrally to the radular sheath some distance 

 forwards. 



The Ventral Retractors are much inferior in strength 

 and number to the dorsal muscles. Like the latter, they 

 are attached both to the floor of the proboscis sheath 

 and to the anterior ends of the lateral odontophoral 

 bands. In fig. 18, the ventral muscles are supposed to 

 be seen through the odontophoral cartilage, all the dorsal 

 muscles and dorsal part of the radula having been 

 removed. It will be seen that on each side there is a 

 bi- or tri-partite muscle, the lateral ventral retractor, 

 which runs from the lateral tongue band, and is attached 

 in front to the sides of the radula (figs. 17 and 18, v.v.m.). 

 In addition to these there are two median ventral muscles 

 (fig. IT, m.v.r.), which lie in close contact with the 

 ventral sheath of circular-muscle fibres (fig. 18, v.e.m), 

 and then run back to be inserted in the same region as 

 the dorsal retractors, the floor of the proboscis. 



