14 



chitinous plates armed with teeth, which are directed up- 

 wards and backwards so as to prevent the return of food 

 which has entered the stomach. 



The stomach (PL II., 5^.) is widest at its anterior 

 end, which extends forward a little distance in front of 

 the opening of the oesophagus, and gradually narrows 

 behind. The anterior wall is furnished with two strong 

 vertical plates, which project backwards into the lumen 

 of the stomach, and are armed with strong spines. Thick 

 bands of striated muscle inserted into these plates, as well 

 as bands inserted into those guarding the opening of the 

 oesophagus, pass down in front of the anterior wall of the 

 stomach, their lower ends being inserted into the labrum ; 

 they enable the creature to depress or raise the chitinous 

 plates at will. The stomach, like the oesophagus, is thin- 

 walled and lined with thick cuticle, toothed ridges of 

 which divide it more or less completely into three 

 chambers, the middle one only containing the food. A 

 pair of thin horizontal plates of cuticle, armed with long 

 setse, shut off a shallow upper portion, in which no food 

 is seen; these plates extend from the anterior end of the 

 stomach to the narrow posterior opening, which their 

 long, backwardly-directed setae help to close. This upper 

 chamber probably serves as a channel to facilitate the 

 distribution of digestive juices through the stomach, its 

 lower wall being formed, in the middle line, only by the 

 long, interlocking setae of the two plates. Behind the 

 opening of the oesophagus runs a deep, longitudinal, 

 ventral groove, divided into two for the first part of its 

 length by a median ridge, and shut oif in front by two 

 pairs of narrow vertical, leaf-like plates of chitin, 

 attached to its sides, and having their free edges fringed 

 with long setae. This groove is cut oft' from the middle 

 of the stomach by overlapping horizontal plates, which. 



