48 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 37 



like edge, though small, are slanted backward, and it is apparent that 

 they aid in pushing food back into the oesophagus. 



A dorsal dissection indicates that there are two sets of muscles con- 

 cerned with the movement of this lobe (fig. i C). They are : 



1. The anterior extensor of the dorsal lobe; a bundle of several 

 fibers arising on the anterior head wall and inserted on the wall of the 

 oesophagus where the lobe joins the oesophagus (fig. i C, D, exdl). 



2. The median retractor of the dorsal lobe ; a large muscle bundle 

 attached to the dorsal body wall, extending forward where it forks 

 to pass around the anterior extensors of the dorsal lobe, each branch 

 being inserted on the posterior median wall of the dorsal lobe (fig. 

 I C, rdl) . 



Laterad of the retractor of the dorsal lobe there are two diagonal 

 muscles which, though not attached directly to the base of the lobe, 

 are closely associated with muscles i and 2 and have considerable 

 effect on the functioning of the lobe. They are: 



3. The lateral dilators of the pharynx originating on the body wall, 

 passing inward underneath the anterior protractors of the feeding 

 claws (aprm) to their insertions on either side of the oesophagus. 

 These muscles resist the pull of the median retractors and are in turn 

 opposed by the action of the circular muscles of the oesophagus 

 (fig. I C,D,ldl). 



On the anterior surface of the head there is a group of muscles 

 directly concerned with the opening and closing of the preoral cavity. 

 These muscles are : 



4. The anterior dilators of the preoral lobes ; muscle fibers arising 

 dorsad on the front of the head; inserted at the base of the lobes at 

 each side of the median dorsal lobe (fig. 2, B, C, D, adl). 



Other muscles concerned with the swallowing of food, though they 

 are not actually attached to the oral lobes, are the following : 



5. The ventral pharyngeal dilators. These are vertical muscles con- 

 sisting of distinct fiber bundles originating on the ventral median 

 body wall and inserted dorsally on the underside of the oesophagus 

 (fig. 2B, C, D,vphd). 



Several groups of muscles in the head consist of flattened fibers 

 lying in sheets close to the body wall. These sheets are very thin and 

 appear almost membranous. These are : 



6. Ventral longitudinal dilators of the oral lobes. These are divided 

 into two bands originating on the ventral body wall, extending for- 

 ward, one band on each side of the ventral pharyngeal dilators and 

 inserted in the walls of the ventral posterior oral lobes (fig. 2 B, C, 

 D, fig. 3 A, z^W). 



