WHEEL-BEARERS -261 



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

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

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

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

 ficies 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 ven- 

 tral side where the passage which I have called the throat, 

 but which is more correctly desig- 

 nated the luccal funnel, enters. 

 Within this muscular bulb are 

 placed two bent organs like ham- 

 mers, called mallei, and a third 

 central table, called the incus. The 

 mallei approach each other dorsally, 

 while the incus is placed toward 

 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 (manubrium) 

 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, con- 

 nected by a thin web of membrane. 



The incus also consists of several distinct pieces. The 

 principal are two stout rami, resting on what appears, when 

 you look at the back or belly of the animal, to be a slen- 



MOUTH OF BRACHIONUS. 



