Messrs. Hancock and Embleton on the Anatomy of Eolis. 5 



which they are placed, partially imbedded in the muscles. They 

 are of an irregularly elliptical form, slightly concave internally 

 and convex externally, and are gradually thinned to a fine edge 

 at their inferior and posterior margins. From the superior mar- 

 gin of each plate near its anterior part projects inwards a trian- 

 gular process, PL I. fig. 9 b and PI. II. fig. 7a ; these processes are 

 united at their apices on the median line by a strong ligament, 

 forming a hinge-like joint or pivot on which the horny plates 

 move easily. Below and a little in advance of these processes 

 project downwards the two large arched cutting-blades, PI. I. 

 fig. 9 c, PI. II. 5, and 7 b ; these blades form the anterior edges 

 of the corneous plates, and end inferiorly in long pointed pro- 

 cesses, w^hich are kept together by muscular insertions. 



The upper surface of the processes for the hinge is divided un- 

 equally by a slight ridge, PI. I. fig. 9 a and PI. II. 7 c, into two 

 parts ; these give attachment to transverse muscles which move the 

 horny plates upon the pivot : the muscle in front of the ridge, 

 PI. I. figs. 6 6*', 7d, 10c, and PI. II. 2 b, is also in front of the pivot, 

 and has the ofiice of closing the jaws ; that behind the ridge, 

 PI. I. figs. Q d,7 e and 10 b, is the opponent of the former and 

 opens the jaws. The latter is much larger than the former and 

 consequently stronger, and extends backwards as far as the oeso- 

 phagus. There is however another transverse muscle, PI. I. 

 figs. Qe, \0d and PI. II. 2c, the duty of which is to assist in closing 

 the cutting-blades. This muscle is seen attached to the edge of 

 the horny plates at their anterior inferior aspect below the cut- 

 ting-blades. The closure of the jaws is further materially pro- 

 moted by a sphincter muscle which forms part of the lips, and 

 will be described further on. 



On the upper aspect of the buccal mass, behind, and partly 

 covered by the transverse muscle that opens the jaws, and run- 

 ning on each side of the oesophagus backwards and then down- 

 wards, is a well-defined muscular layer, PI. I. figs. 5 a, 7 /and 9 e, 

 having its origin from the inner border of the horny plates. The 

 fibres which arise the furthest forward form the inner edge of 

 each muscle, and unite on the median line immediately behind 

 the oesophagus ; those which come off" behind these pass parallel 

 to them, and are united also on the median line at points suc- 

 cessively further behind and below the former, and the fibres 

 which are last in origin are prolonged and become lost upon the 

 under aspect of the buccal mass. All the fibres of this muscular 

 layer, besides uniting with each other, are attached by their an- 

 terior surface to the muscles of the tongue upon which they lie. 

 One ofiice of these muscles appears to be to pull forwards the 

 oesophagus so as to close its orifice; their principal function we 

 will explain when we come to the tongue. 



