6 JEAN DAWSON. 



of the mouth is formed by the large underlying tongue muscle 

 (Fig. i, 0). The tongue as a whole is a large, long cone which 

 stretches from the mouth opening to the ventral anterior portion 

 of the pericardium to which it is attached by its apex. Thus the 

 tongue underlies the whole mouth cavity, pharynx, and water 

 tube. When the tongue is pushed forward so that its lobes 

 occupy the flaring anterior part of the mouth cavity, its lateral 

 lobes fall apart (Figs. 5, 6) and there is left between them a pas- 

 sage-way which connects the cavity of the oral funnel and that 

 of the mouth. This passage-way, which then has a cross section 

 equal to that of the mouth cavity at its middle, diminishes rapidly 

 in diameter as the tongue is drawn back into the narrower part 

 of the mouth cavity until at last it is completely closed before the 

 tongue has reached the posterior end of the semiannularis car- 

 tilage. The end of the tongue is thus seen to act as a piston 

 working with a short stroke back and forth in the mouth cavity, 

 the walls of which form the cylinder v/hile the free end of the 

 tongue forms the piston-head and its lateral lobes the valves. 



B. Pharynx. - - Passing out of the mouth cavity through the 

 small opening ventral to the semiannularis muscle, the food enters 

 the cavity of the pharynx. This cavity is somewhat irregular 

 (Fig. i, </). Its dorsal anterior portion is wedge-shaped in lon- 

 gitudinal section and extends forward over the semiannularis 

 muscle and semiannularis cartilage. This portion of the pharyn- 

 geal cavity thus lies dorsal to the posterior portion of the mouth 

 cavity. The remainder of the pharyngeal cavity is about 2.5 

 cm. in length from where it joins the mouth cavity to where it 

 opens into the oesophagus and water tube. Instead of having a 

 wall whose mucous membrane lies directly upon the tongue 

 muscles on the ventral side and against the cartilage on the 

 dorsal side, as in the case of the mouth cavity, there is, in the 

 wall of the pharyngeal cavity, a thin layer of muscular tissue, 

 the pharyngeus muscle intervening between the mucous mem- 

 brane and the outer layers of muscle or cartilage. The pharyn- 

 geus (Fig. 4, <:), which is fully described by Fiirbringer (1875), 

 entirely envelops the pharyngeal cavity and ends on a raphe 

 on the mid-ventral line of the cavity. It is in contact with 

 the mucous membrane except in the ventral lateral part of the 



