WHEEL-BEAEEES. 269 



and, working vigoronsly, bruise it down with a ham- 

 mer-like action upon a sort of central tahle. After this 

 process has gone on for a few minutes the green mass, 

 less perfectly defined than before, slips through a nar- 

 row postern-gate, along a short narrow alley, into the 

 digesting stomach. 



But what sort of a mouth is this ? It is inclosed 

 within the tissues of the body, not very far from its 

 centre, so that no part of it comes into contact with the 

 external water, or even approaches any part of the su- 

 perficies of the body. It has been usual to call the great 

 hemispheric bulk in which the symmetrical hammers 

 work so vigorously, a gizzard, but it is a true mouth, 

 and the hammers are true jaws. 



This form of mouth is termed a mastax ; it consists 

 of a dense but transparent muscular mass, forming 

 three lobes at its lower part, deeply cleft at the front of 

 its ventral side where the passage which I have called 

 the throat, but which is more correctly designated the 

 iuccal funnel, enters. Within this muscular bulb are 

 placed two bent organs like hammers, called mallei, 

 and a third central table, called the incus. The mallei 

 approach each other dorsally, while the incus is placed 

 towards the ventral side, its stem pointing obliquely 

 away from the centre. 



Each malleus consists of two portions, united by a 

 free but powerful hinge-joint. The lower joint {ina/nu- 

 hrium) is shaped somewhat like a shoulder-blade ; and 

 the upper joint (uncus) is set on at nearly a right angle 

 to it, but is capable of considerable change of direction 

 by means of its hinge. It consists of five or six finger- 

 like teeth, connected by a thin web of membrane. 



The incus also consists of several distinct pieces. 



