128 MORPHOLOGY OF INVERTEBRATE TYPES 



dite. The main body of the propodite is called the hand, the 

 dactylopodite the movable finger, while the immovable finger is 

 simply a process of the propodite. The first pair of pereiopods, 

 the so-called great chelipeds, are especially powerful. Their 

 function is to hold and crush the food. It is a common occurrence 

 among crabs and lobsters that the left and right great chelipeds 

 are not alike. In the lobster one claw is heavy, is used for 

 crushing the food, and its fingers for this reason have a row of 

 rounded tubercles. The other claw is more slender, is used for 

 seizing and holding the prey, and its fingers have a row of sharp 

 teeth. All pereiopods are easily broken off by the animal if 

 held fast by an enemy. The breaking plane is always between 

 the baso- and ischiopodite. Anatomically this is due to a 

 special mechanism and it is interesting to remember that the 

 part of the limb remaining intact consists of the two joints of 

 the protopodite. The relation of the pereiopods to the genital 

 openings will be described later. 



The following six pairs of appendages belong to the abdomen 

 and are called pleopods. The first pair is uniramous. In the 

 female it is quite small, in the male it is developed as a pair of 

 stylets for copulation. The second, third, fourth and fifth pairs 

 of pleopods are biramous and very much alike in structure. 

 They consist of a single- jointed protopodite and flat exopodite 

 and endopodite, both the latter fringed with tactile hairs. These 

 pleopods are used for forward swimming and, in the female, for 

 holding and aerating the eggs. The last pair of pleopods is at 

 the same time the last pair of all appendages. These pleopods 

 belong therefore to the twentieth somite and form together with 

 the anal somite the tail fan. They are usually called uropods. 

 The exopodite is two- jointed. Both the endopodite and exo- 

 podite are in the shape of broad and flat paddles fringed with 

 tactile hairs. 



Muscular system. It is clear that a hard, segmented 

 exoskeleton must possess a segmented muscular system of a 

 different type from that of segmented worms in which the in- 



