88 GENERAL ZOOLOGY 



be chewed by the mandibles. The mouth leads to a narrow 

 esophagus extending dorsally to about the center of the head, 

 where it turns posteriorly to dilate into the crop. The lat- 

 ter is lined with a cuticular membrane armed with toothlike 

 projections for completing the process of mastication begun in 

 the mouth. Alongside the crop are branched salivary glands 

 whose ducts run forward to empty into the mouth. Next comes 

 the stomach surrounded by a set of six to eight double cone- 

 shaped pouches, known as caeca, which secrete a digestive fluid. 

 The posterior limit of the stomach is marked by a large number of 

 Malpighian tubules, excretory in function, which enter the diges- 

 tive tube at the junction of the stomach with the intestine. The 

 latter passes back to terminate at the tip of the abdomen in an 

 anal opening. The rectum is an enlargement in the intestine 

 near the anus where fecal matter accumulates until excreted. 



Digestion and absorption in the grasshopper take place in the 

 stomach and intestine, whence the products of digestion pass into 

 the body fluid filling the hemocoel. The body fluid and the blood 

 in insects is the same, and is pumped by the heart to all parts 

 of the body, supplying the tissues in this way with nutrition and 

 at the same time ridding them of the waste products of metabo- 

 lism. Undigested food and excretory products are expelled 

 through the anus. 



Vertebrate Alimentary System.— The alimentary canal of 

 vertebrates begins with a mouth, opening into an oral cavity, 

 continues as a tube, regionally differentiated, and terminates in a 

 rectum that opens into a cloaca, as in the frog, or directly to the 

 outside, as in the majority of mammals. 



Oral Cavity. — The lining of the oral cavity as well as part of the 

 rectum is ectodermal in origin, i.e., it is derived from the primitive 

 outer covering of the vertebrate embryo. The lining of the 

 remainder of the alimentary canal is formed of endoderm, which 

 for the present may be described as an internal germ layer. 

 Briefly, the oral cavity develops as an invagination of the head 

 ectoderm, producing an indentation, that deepens until it comes 

 in contact with the wall of the pharynx. The ectoderm of 

 the oral cavity fuses with the endoderm of the pharynx, after 

 which the fused layers break down to form a passageway. The 

 posterior opening of the alimentary canal is formed in a similar 

 way. From this it follows that the lining of the oral cavity 



