21 



develop from paired fuudanients in the embryo (Plate 

 VII., tig. To), almost all traces oi' such an origin become 

 obliterated in the completed structure. Its distal 

 extremity, however, is seen to consist of two halves, which 

 are in close contact with one another along the mid- 

 ventral line without being fused together (fig. 12, Ih.). 

 Each half is to be regarded as the equivalent of an 

 undifferentiated galea and lacinia. Labial palpi are 

 present in the embryo, but they subsequently become 

 resorbed. The remaining portion of the labium extends 

 from the suture to the anterior margin of the pro- 

 thorax, and would appear to be the equivalent of an 

 undifferentiated mcntum, sub-mentum, and gula. 



While feeding is taking place, both the mandibles 

 and first maxillie are protruded from their pockets for 

 the purpose of seizing the food, and they can be seen with 

 the aid of a strong lens to be projecting a short distance 

 through the mouth. By means of the retraction of these 

 parts, aided by the movements of the labrum and labium, 

 the food is taken into the mouth. Mastication is chiefly 

 performed by the combined action of the mandibles and 

 first maxillse. The part played by the maxillulse appears 

 to be a more or less passive and subsidiary one, for, since 

 they are closely applied to the lingua, they possess but 

 little power of movement. When the food is finely 

 comminuted, it is sucked up into the cBsophagus by means 

 of the alternate contraction and expansion of the radial 

 and circular muscles of the pharynx. 



1. The Digestive System. 



The digestive system is divisible into three well- 

 defined regions, viz., the fore-gut, the mid-gut or stomach, 

 and the hind-gut. It passes in a perfectly straight course 



