OP ARTS AND SCIENCES. 97 



rynx. They are very deeply stainable, have many processes, and 

 almost completely fill the space between the pharyngeal glands and 

 the dorsal wall of the body. 



Thus the pharynx forms a highly specialized organ. It is, more- 

 over, in almost constant use for purposes of prehension. It is used 

 both in seizing the food and in locomotion. When it is extruded, the 

 mouth opening is rounded and the pharynx has a cylindrical form. 

 The protrusion is initiated by the action of the protractor muscles 

 already described, and completed by the pressure communicated through 

 the fluids of the body to the walls of the partly extruded pharynx, 

 when the body walls are forcibly contracted. When it is extruded, 

 the strong vibratile cilia of its walls are easily seen. They must help 

 to render the extruded pharynx of use as a sucking disc, a purpose to 

 which the mucous glands are doubtless also subservient. When ap- 

 plied to any object, the extruded portion may be rendered concave on 

 its outer surface by the action of the muscular fibres in its walls, 

 and those which pass from it to the body walls above ; thus it becomes 

 an effective sucking-disc. 



The cesophagus begins at the septum between the fifth and sixth 

 segments, and ends at that between the eleventh and twelfth. A 

 slight but sudden increase in the size of the tube separates it from 

 that part of the pharynx which immediately precedes it. The pres- 

 ence of liver cells also marks its beginning. It is of uniform diameter 

 until it reaches the ninth segment, although it may be somewhat con- 

 voluted. In the eighth, ninth, and tenth segments it is much swollen, 

 while in the eleventh it is of the same diameter as in those preceding 

 the enlargement. 



It is marked off from the intestine following it by (1.) a sudden 

 large increase in the size of the tube near the partition between the 

 eleventh and twelfth segments; by (2.) a difference in the length of 

 the cilia in the two parts ; by (3.) a difference in the brown drops in 

 the liver-cells of the two regions ; by (4.) a difference between the 

 epithelial cells, as may be seen in sections ; and by (5.) the absence, in 

 most cases, of epithelial glands in the oesophagus, and their presence 

 in the intestine. 



While the cilia of the oesophagus are so long as to almost fill its 

 lumen, those in that part of the intestine immediately following it are 

 very short, or perhaps in some cases entirely absent. 



In a portion of the oesophagus, viz. in the enlargement of the ninth 

 and tenth segments, the lining epithelium presents a peculiar appear- 

 ance (PI. II. fig. 17, en'.). It no longer exists in the form of a simple 



VOL. XX. (n. S. XII.) 7 



