It seems at first sight strange to include in the same category as "limbs" or "appendages" organs which differ so much in form and function as do the swimmerets, the walking legs, the jaws, and the antennæ. Nevertheless it can easily be demonstrated that all of them are constructed on the same general plan, and arise in the embryo from rudiments which are, for the most part, exactly alike. This is expressed in technical language by saying that the appendages of the whole series are homologous with one another. A full discussion of this interesting fact would require more space than can be devoted to it here, but a few examples may be given to illustrate what is meant by the "serial homology" of the appendages in Crustacea.
Fig. 3—Third Maxilliped of Lobster. (From British Museum Guide.)
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If one of the swimmerets be detached from the third abdominal somite, it will be seen ([Fig. 2]) to consist of a stalk, known as the protopodite, bearing two branches, of which that on the outer side is called the exopodite, and that on the inner side the endopodite. The protopodite consists of two segments, the first very short, and the second much longer. It can easily be seen that the side-plates of the tail-fan (the middle plate, as already mentioned, is the telson) are simply the swimmerets of the sixth abdominal somite. They are much larger than the other swimmerets, and have the endopodite and exopodite broadened out into large plates; while the protopodite is very short, and not divided into segments.
Fig. 4—Walking Legs of Lobster
A, Of first pair; B, of third pair