ANIMALS WITH JOINTED LEGS— THE ARTHROPODS 



189 



by a gizzard which is lined with teeth made of chitin that grind and 

 pulverize the food. The food then passes out into a pair of large diges- 

 tive glands, where digestion and absorption take place. These glands, 

 in addition to secreting the enzymes of digestion, thus serve the same 

 function as the small intestine in the higher vertebrates. The residue 

 is then passed into the posterior, or pyloric portion, of the stomach and 

 on into the intestine. There is little absorption of food from the in- 

 testine because most of it is lined with chitin through which the digested 

 food cannot pass. The intestine, therefore, serves a similar function to 

 the large intestine of the higher vertebrates ; it is primarily for storage 

 and transportation of the indigestible residue of the food to the anus 

 for elimination. 



Gastric mill 



Heart 



Ostium of heart 



Intestine 



Bladder 



Anus 



Oreen gland 



Oesophagus 

 Mouth 



_ ionc portion of Stomach 

 Cardiac portion of stomach 



Fig. 13.6 Diagram showing the location of the organs of digestion and excretion in 

 the crayfish. The position of the heart is also shown. 



Respiratory System. The respiratory organs consist of a series of 

 gills which lie in the branchial chambers on either side of the thoracic 

 region of the body. The chambers are formed by an overlapping por- 

 tion of the exoskeleton, the carapace. Water is kept flowing through 

 the chambers by the action of the bailer, which is attached to the second 

 maxilla, that is continually bailing the water out at the anterior end so 

 that it keeps flowing in at the posterior end. This keeps the gills bathed 

 in fresh water so that oxygen may be absorbed and the carbon dioxide 

 given up. The process is aided by movements of the gills themselves ; 

 most of them are attached to appendages, and, when the appendages 

 are moved on the outside, the gills are moved in the chambers. 



Excretory System. A person who knows how to eat a lobster does 

 not stop when he has finished the large muscles in the "tail." There are 

 many of the internal organs that are good eating, but a pair of green 

 glands at the anterior end of the cephalothorax should be avoided be- 

 cause they have a very bitter taste. These are also present in the lob- 

 ster's first cousin, the crayfish, and are located in the ventral part of the 



