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BULLETIN OF THE BUREAU OF FISHERIES. 

 Table 4. — ^The Body Segments and Appendages of the Lobster with 



In the typical thoracic leg (pi. xxxviii) the endopodite is divided into 5 segments, 

 which, with the two divisions of the protopodite, give the limb 7 podomeres, numbered'' 

 and named from base to apex as follows: (i) Coxa or coxopodite, (2) basis or basipo- 

 dite, (3) ischium or ischiopodite, (4) meros or meropodite, (5) carpus or carpodite, (6) 

 propodus or propodite, and (7) dactyl or dactylopodite. These successive segments 

 are articulated to the body and to one another by soft membrane and usually by hinge 

 joints which limit the movements of each to a single plane at right angles to the articular 

 axis, or to the line joining the two hinges; each segment, with the exceptions to be 

 noted later, is actuated by opposing muscles, a larger flexor and a smaller extensor, the 

 fibers of which are implanted over the hard shell of their respective segments and are 

 inserted on strap-shaped tendons which react on the distal podomere (fig. i, pi. XLi). 

 The tendon is derived from an ingrowth or flattened pocket of interarticular membrane 

 (fig. 2, mb., pi. XLi, and fig. i,tp., pi. xLni), and is sometimes closely united to the shell of 

 the distal segment. Each joint or articulation is therefore crossed by tendons which 

 belong to the proximal podomere and pull on the distal one. 



In the successive somites of the tail the axes of articulation are all parallel, and 

 at right angles to the longitudinal axis of the body so that movement is limited to the 

 vertical plane. In the appendage, on the other hand, the direction of the axis of articu- 

 lation varies in successive podomeres (see figs. 6 and 7) ; moreover the initial direction of 

 movement of the base of each limb, which depends upon the angle which its articular 

 axis makes with the long axis of the body, varies greatly from head to tail (135° in the 

 mandibles, about 55° in the great chelipeds, and 90° in the swimmerets). Accord- 

 ingly each segment acts as a lever of the third order, and the successive thoracic limbs 

 are capable of universal movement, and in a variable field. By reference to figures 



a This order seems preferable to the reverse, which is sometimes adopted, since the protopodite has been less modified 

 than either of its branches, and we thus avoid the ambiguity of speaking of the seventh segment of a pleopod or of an 

 antenna 



