376 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



Those appendages of the thoracic region which are 

 confined to a locomotor function have the form of an 

 elongated jointed bar, consisting of seven joints, which 

 have received the following names : the first two, 

 which appear to correspond to the protopodite of the 

 typical appendage, are called coxopodite and basi- 

 podite ; the remaining five, which may be supposed to 

 represent the endopodite, are 

 ischio, mero, carpo, pro, and 

 dactylo-podites. 



While these ambulatory 

 appendages move in a plane 

 which is parallel to the long 

 axis of the body, those of the 

 abdomen swing backwards a ad 

 forwards, owing to the fact 

 that the abdominal segments 

 are, unlike those of the thorax, 

 which is invested by the hinder 

 part of the great carapace, 

 capable of being moved on one 

 another. This movement of 



segments is brought about by two great bundles of the 

 muscle, which lie respectively above and below 

 the intestine, and are attached to the tergal and 

 sternal plates of the separate segments. It follows, 

 from their mode of attachment, that the contraction 

 of the upper muscles straightens out or extends the 

 abdomen (tail), and that a contraction of the lower 

 muscle tends to bend in or flex the same parts. An 

 alternate contraction and relaxation of these muscles 

 tends therefore to an alternate bending in and 

 straightening out of the " tail," and therefore to 

 repeated beats of the water, by means of which the 

 crayfish or lobster is driven through it. In the 

 performance of this locomotor action the " tail " is 

 greatly aided by the modification of the appendages of 



Fig. 159. Third Abdominal 

 Segment of the Lobster. 



f, Tergura (dorsal piece) ; , 

 sternum (ventral piece); pi, 

 pleuron ; p, protopodite ; ex, 



exopodite ; en, endopodite. 



