34 Elementary Zoology. 



are powerful swimming organs, at first sight very different from 

 the comparatively feeble limbs that we have been studying; 

 but they are plainly reducible to the same elements. There 

 is the protopodite, of two joints; then both exopodite and 

 endopodite, of which the former only is plainly two-jointed. 

 The joints, moreover, are strong and flattened plates, not 

 annulated flagella-like outgrowths (Fig. 15, F). 



Working forwards from the first pair of the abdominal 

 appendages, the next five pairs (Fig. 16) are ambulatory 

 appendages : they are used for walking ; and the fact that 

 there are five has given the special group of arthropods to 

 which the crayfish belongs the name of Decapoda. As their 

 function is different from that of the swimming swimmerets, 

 so their structure is also different. If we take one of the last 

 of these limbs, it is seen to be made up of seven joints of vary- 

 ing lengths, which are thus named, commencing with that 

 which articulates with the body : coxopodite, basipodite, ischio- 

 podite, meropodite, carpopodite, propodite, dactylopodite. These 

 joints are to be found in all the five pairs of ambulatory 

 limbs ; but in the three first of these appendages the propodite 

 is lengthened out into a process which lies parallel with the 

 dactylopodite, and forms with it a pair of " pincers." In 

 the first pair of ambulatory limbs these are especially well 

 developed, and the whole appendage is larger, being called 

 the chela. It is not, at first sight, obvious how these ambu- 

 latory limbs can be brought into line with the abdominal 

 swimmerets. They are uniramous, and not biramous. If it 

 be supposed that they are uniramous through the loss of one 

 branch, which branch has been lost, the exopodite or the 

 endopodite ? The crayfish itself supplies no answer to this 

 question ; but its near ally, the lobster, does, a fact which 

 shows the necessity of a comparative study for the unravelling 

 of such problems. 



When the young lobster is newly hatched, its thoracic 

 appendages are in the " schizopodous " condition, as it is 

 termed, i.e. each is biramous ; later on the outer branch dis- 

 appears, and that which is left is thus the endopodite. It 

 may be inferred, then, on account of the detailed likeness 



