STRUCTURE OF ONYCHOPHORAN HEAD BUTT 



47 



to form an organ to help in the swallowing of food. Though it is 

 equipped with powerful muscles and has a row of external spines, in 

 these respects resembling somewhat the epipharynx of some insects, 

 it cannot in any way be considered as a labrum. Neither is it a tongue 

 in the sense that the hypopharynx of chewing insects is a tongue. It 

 simply is one of the circumoral ring of lobes, though much larger than 



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Fig. 2. — Lateral dissections of tlie head. 



A, vertical dissection with brain in place, dorsal body wall removed. B, 

 median dissection with brain cut. C, same with oesophagus removed. D, same 

 with brain removed. 



Antn, antenna! nerve ; Apcl, apodeme of feeding claws ; Br, brain ; Cof, cir- 

 cumoral folds ; Fcl, feeding claws ; Fein, nerve of feeding claw ; Hypo, Hypo- 

 cerebral organs ; Occl, occular lobe ; Papn, papillar nerve ; adl, anterior dilators 

 of the oral lobes ; aprni, anterior protractors of the feeding claws ; exdl, anterior 

 extensors of the dorsal lobe ; drm, dorsal retractors of the feeding claws ; dv, 

 dorsoventral muscles; vld, ventral longitudinal dilators of the oral lobes; vphd, 

 ventral dilators of the oesophagus; vnn, ventral retractors of the feeding claws. 



any others in the preoral cavity, and is here called the dorsal lobe 

 (Dl). 



Now let us examine this structure more closely. When the mouth 

 is closed it is apparent that this dorsal lobe closes the opening by 

 fitting tightly within the circle of other lobes. When the animal relaxes 

 the muscles of the mouth region, the mouth opens wide and the dorsal 

 lobe may be pushed out and retracted. The spines on its lower keel- 



