20 ZOOTOMY. 



XII. Remove the eye, and dissect away the muscles, 

 cartilages, &c. (§ 62), which form the left side 

 wall of the mouth-cavity, first passing a probe 

 into the latter from the oral aperture to guard 

 against cutting too deeply. Also open the oesopha- 

 gus and the respiratory tube by a longitudinal 

 incision along the left side of each. The following 

 points can now be made out : — 



64. The almost tubular oral cavity (Fig. 4, ot), com- 

 municating with the oral funnel by the very narrow oral 

 aperture. 



65. The oral caecum, a blind pouch of the mucous membrane in 

 about the middle of the mouth-cavity, on the dorsal side (Fig 4, m) ; 

 it is continued posteriorly into a longitudinal groove. 



66. The aperture of the oesophagus (Fig. 4, between 

 ;« and (es), at the hinder end of the oral cavity, and im- 

 mediately beneath it, that of the respiratory tube. 

 Between the two apertures the mucous membrane forms a 

 sort of horizontal shelf, produced into five finger-like pro- 

 cesses in P. fluviatilis, and in P. marinus into two 

 blunt processes supported by small cartilages. The entrance 

 of the respiratory tube is also guarded by two lateral flaps of 

 mucous membrane, the vela. (Fig. 4, vl). 



67. The spirally arranged ridges into which the mucous membrane 

 of the gullet is produced. 



68. The extent of the respiratory tube, and the internal 

 branchial apertures of the right side. 



XIII. Open the pericardium by carefully removing the 

 greater part of its left wall : note 



69. The auricle (Fig. 4, au), occupying the whole of the 

 left side of the pericardial chamber, and therefore the only 

 part of the heart seen at first. It is strongly convex on its 



