INTERNAL ANATOMY. 67 



the mouth and the maxillary lip ; it forms the floor of 

 the pharynx ; it is usually more chitinized than the 

 upper half-tube, and is either permanently fixed or 

 else attached to very short muscles, as in the Oriba- 

 tidse. The upper half -tube is more or less flexible, 

 and forms the roof of the pharynx ; it is by the move- 

 ments of this roof that suction and swallowing are 

 effected; into its dorsal surface a number of paired 

 muscles are inserted, generally by a tendinous attach- 

 ment, which is often long and single, as in Bdella, or 

 may be a line of several very short tendons, as in Thy as : 

 these muscles are conspicuous, and may, in good 

 sections, be easily traced upward or upward and 

 backward; and it will then be found that they arise 

 from the lower surface of the roof of the rostrum 

 (PL A, fig. 5, rr), upon the upper surface of which 

 the mandibles rest ; this roof is called " Chitin 

 Boden " by Schaub, who treats it as a floor which the 

 mandibles repose on, and its hinder part, which is 

 sometimes thickened, is called " Chitin Brucke " by 

 Henkin. These muscles (fig. 5, m I p) I have called 

 the levator tecti pharyngis or dilat ores pharyngis muscles ; 

 when they contract they draw the roof of the pharynx 

 upward, leaving a partial vacuum between it and the 

 floor of the organ ; into this vacuum the food con- 

 tained in the mouth rushes ; it is prevented from 

 returning by some sort of valve or constrictor muscles ; 

 the dilatores pharyngis are then relaxed, the upper 

 half-tube is allowed to descend upon the lower one, 

 partly often by its own elasticity, but more by the 

 action of occlusor pharyngis muscles (mop). The food 

 is thus forced backward into the oesophagus, and 

 finally into the ventriculus. There is considerable 

 variety in the occlusor muscles, but probably the 

 commonest form, particularly in such genera as Trom- 

 bidium and Bdella, is that between each distensor muscle 

 and the one before it, a short, transverse, horizontal 

 muscle passes across and above the pharynx ; it is 

 attached at each end to one edge of the two half -tubes 



