ALIMENTATION 87 



posteriorly. A ciliated layer of ectoderm covers the outside, and 

 an inner layer of endoderm lines an alimentary tract. Between 

 the two are muscle tissue and other organs derived from a third 

 germ layer, the mesoderm. Euplanaria is, therefore, triplo- 

 blastic. The opening into the alimentary tract is on the ventral 

 surface of the body near the middle through which a muscular 

 tube, the pharynx, can be thrust like a proboscis. Inside the 

 body the pharynx connects with an intestine consisting of three 

 main trunks, one forward and two backward, from each of which 

 numerous smaller branches, ending blindly, penetrate among the 

 tissues. Digestion, both intracellular and extracellular, takes 

 place in the intestine, which also serves to distribute the products 

 to the tissues by means of its collateral branches. 



OP 



ov 



M SG y U 



Fig. 53. — Internal anatomy of a grasshopper. A, anus; C, crop; CM, caecum; 

 E, eggs; G, supresophageal ganglion; H, heart; 1, intestine; L, labium; M, 

 mouth; O, esophagus; OP, ovipositor; OV, oviduct; R, rectum; S, stomach; SG, 

 salivary gland; U, Malpighian tubules; V, ventral nerve cord. (After Packard, 

 Textbook of Entomology.) 



Complete Alimentary Canal. — The digestive organs of an 

 insect like the grasshopper display a considerable advance in 

 complexity over the preceding forms, and have many features in 

 common with the vertebrate type of system. The alimentary 

 canal of the grasshopper is a tube open at both ends, beginning 

 with a mouth and terminating in an anus; a type that is known as 

 a complete alimentary canal (Fig. 53). The tube lies in a cavity 

 of the body, the hemocoel, which is filled with blood. The mouth 

 is provided with organs of mastication : a labrum, or upper lip ; a 

 labium or lower lip ; and between them a pair of mandibles and a 

 pair of maxillae. The mandibles are the principal chewing 

 organs, and both they and the maxillae operate by a side-to-side 

 movement, instead of up and down as with the jaws of man. 

 The maxillae aid the lips in manipulating the food so that it can 



