SOFT PARTS AND MECHANISM OF THE PROBOSCIS. 397 



vesicle (e) in the oral sucker. Both trunks give off numerous 

 cylindrical and vesicular branches in their course. 



As I have shown, there can be little doubt the air in the 

 tracheal vessels of the thorax and abdomen is maintained at a 

 higher pressure than that of the external air, and all the large 

 membranous tracheal trunks of the anterior thoracic and 

 cervical region are surrounded by loops of muscle-fibres capable 

 of compressing them and forcing air into the head capsule. 

 The mere opening of the valve in front of the jugum probably 

 suffices to fill the air-vessels of the proboscis; when the valve 

 is closed, the air has still a passage from the air-sacs of the 

 proboscis through the tentorial rete, by which they are prob- 

 ably emptied, as the organ is drawn into the head capsule, 

 by the contraction of its retractor muscles. 



Kraepelin at one time supposed that the frontal sac is con- 

 cerned in adapting the size of the head capsule to its contents, 

 and that its expansion accompanies the exsertion and its con- 

 traction, the withdrawal of the proboscis. In his finished 

 memoir [70] he states that he no longer held this view. I am 

 myself inclined to return to it, as a number of muscle-fibres 

 cover the cephalic surface of the frontal sac ; two bundles also 

 arise from it and descend to be inserted into the pharyngeal 

 extremity of the oesophagus. The contraction of these muscles 

 assists in the retraction of the proboscis, and can hardly fail to 

 diminish the size of the frontal sac by pressing together its 

 folds ; if this is so, it would assist in drawing air through the 

 tentorial rete mirabile into the upper part of the head capsule, 

 thus emptying the tracheal sacs of the proboscis and assisting 

 in its retraction. 



d. Function of Oral Lobes. 



The Inflation of the Oral Sucker. The fact that the oral sucker 

 is flaccid when folded and at rest, and turgid when in action, 

 has long been well known, but authorities are divided as to the 

 manner in which this change is effected. Kraepelin ascribes it to 

 an injection of blood into the organ, but Gleichen, Macloskie, and 

 Dimmock held that the distension of the lips is effected by the 



