130 MORPHOLOGY OF INVERTEBRATE TYPES 



tegument of the body is soft and elastic. On the whole the 

 muscular system is too complicated to be dealt with here, but 

 some of the most important muscles may be mentioned. The 

 extension of the segmented abdomen is accomplished by mus- 

 cles attached to the tergites, the flexion by more powerful 

 muscles attached to the sternites. The thoracic appendages are 

 moved by muscles arising from the endoskeleton and ending in 

 the coxopodites. A flexor and an extensor is present in every 

 joint of the appendages for the next joint, except of course in the 

 dactylopodite which is the last joint. 



Digestive system. The V-shaped mouth is ventral in 

 position and is provided with two lips. The upper lip or labrum 

 has a median keel on both its surfaces, dividing each surface into 

 two concave areas. The lower lip or metastoma is bifurcated. 

 Between the two lips at the sides of the mouth are the powerful 

 mandibles and behind the mouth the maxillae and maxillipeds 

 described above and used as mouth parts in passing the food. 

 The mouth leads directly into a short cesophagus which opens 

 into the stomach. The stomach is a large and complicated organ 

 forming the last section of the foregut. It is divided into three 

 regions, the cardiac sac, the gastric mill and the pyloric sac. 

 On each side of the stomach is a large gastrolithic plate. The 

 gastric mill in which the food is ground into a fine pulp contains 

 a median tooth and lateral teeth. These teeth are the inner 

 projections of special ossicles. The wall of the stomach has a 

 regular framework of articulated ossicles some of which will be 

 mentioned in the instructions. The grinding movement of the 

 gastric teeth is accomplished by the action of special muscles. 

 The anterior gastric muscles are attached to the so-called pro- 

 cephalic lobes of the tergite of the first cephalic somite. The 

 posterior gastric muscle is attached to the carapace at the cervical 

 groove. The food passes from the cardiac sac into the grinding 

 mill separated from the former by a valve. Here it is ground 

 fine and delivered into the pyloric sac. Food that is still too 

 coarse returns to the mill to be reground. Parts that cannot be 



