282 



THE MAINTENANCE OF THE INDIVIDUAL 



fish, insect larvae, snails, and decaying organic matter. The max- 

 illipeds and maxillae around the mouth are used to hold the food 

 while the mandibles crush it into small pieces that are then passed 

 into the esophagus. The large stomach contains a series of chitinous 

 ossicles, forming the gastric mill, which grinds the food. When the 



food has been broken up 



bloctde-r: 



mancLiblsS 

 and. mou-th- 



Gsophagixs —■ r 



sixbesophodeal- " 

 ganglior^ 



-«"J 



Supruesopba6ea.l 



g-a rig lion. * 



circumagophcxSeol 

 conn<activ<e.s ^ 



grinding stomach 



16 



sbemal 

 artsry" "" 



opeTiing" 



vas d&^&rsns^ 



r-i-estis 

 ..'haor-t 

 --ostium 



...dorsal artery 



:I1 



12 



iJ.-inLesLine. 



sufficiently, it passes 

 through the strainer into 

 the pyloric chamber, where 

 the digestive glands or 

 "liver" empty their secre- 

 tions through hepatic ducts. 

 These glands secrete en- 

 zymes which digest both 

 proteins and fats. From 

 this chamber the dissolved 

 food passes into the in- 

 testine where nutritive ma- 

 terial is absorbed through 

 the intestinal wall. 



Digestion in Higher 

 Animals 



^3 



anus— w 



Sagittal section of crayfish showing diges- 

 tive system. 



In the vertebrate series 

 the parts of the digestive 

 systems are analogous and 

 even homologous with some 

 invertebrate structures. 

 All except the lowest and 

 parasitic types of inverte- 

 brates are characterized by 

 an alimentary canal. Dif- 

 erences which occur in the digestive tracts of vertebrates are largely 

 attributable to the different kinds of food handled by different types 

 of systems. Carnivores digest their foods more rapidly than herbi- 

 vores and so can get along with a shorter alimentary tract. 



Methods of Increasing Digestive Surfaces 



One of the first problems in digestion is the production of an ade- 

 quate absorptive surface. Greater digestive surfaces may be prO' 



