4o8 CHARACTERS OF INVERTEBRATE ANIMALS 



make up an elastic framework on which are borne two large 

 lateral and one median tooth, and by the action of appropriate 

 muscles these can be brought together so as to effectually chew 

 anything that happens to be between them. The cavity of the 

 pyloric part of the stomach is narrowed, and numerous inter- 

 lacing bristles project from its walls, constituting a very effective 

 " strainer ", which prevents any but finely-divided particles from 

 passing back into the short mid-gut. The lining of this part 

 of the food-tube is soft, and a pair of large digestive glands, 

 commonly called the liver, open into it. These organs are 

 physiologically equivalent to liver and pancreas of a Vertebrate 

 (see pp. 2,'] and 38). The hind-gut or intestine is a narrow tube 

 continuous with the mid-gut, and possessing a firm lining raised 

 up into longitudinal ridges. 



The Circulatory Organs (fig. 350) conform to the Arthropod 

 type already described in dealing with the Cockroach (p. 348), 

 but the heart, instead of being a long slender tube, is a short 

 broad sac, possessing only three pairs of valvular apertures, and 

 suspended in the blood-containing pericardial cavity by means of 

 fibrous cords. It is systemic (see pp. 308 and 348), i.e. contains 

 purified blood, which it pumps through delicate branching arteries 

 to the body at large. Sooner or later these arteries communicate 

 with irregular spaces which ultimately open into a large sternal 

 sinus running along the lower part of the body just within the 

 body -wall. Meanwhile the blood has become impure by loss 

 of much of its oxygen and receipt of carbon dioxide as a waste 

 product. It therefore passes to the gills for purification, and, when 

 this has been effected, is carried to the pericardial cavity, whence 

 it passes into the heart through the valvular openings with which 

 that organ is provided. 



The Excretory Organs, by which nitrogenous waste is removed 

 from the blood, consist of a pair of antennary- or green-glands, 

 situated in the front part of the head and opening on the bases 

 of the antennae. Each is essentially a coiled tube, possibly equi- 

 valent to a nephridium (see p. 401). 



The Muscular System, is complex. The largest muscles are 

 to be found in the tail, which is the organ by means of which 

 the lobster is able to swim backwards through the water with 

 great rapidity. Powerful flexor muscles, lying below the gut, 

 bend the tail down and enable it to give its effective stroke, while 



