The carapace which covers the upper side of the head and thorax is not formed, as might be thought, simply by the terga of the somites becoming soldered together. This is shown by a comparison with certain shrimp-like Crustacea (_Mysidacea_) in which the carapace arises, like a fold of the skin, from the hinder edge of the head, and envelops, like a loose jacket, the distinctly segmented thorax. In the Lobster this fold has become adherent to the thoracic somites down the middle of the back, but at the sides it hangs free, enclosing on each side a cavity within which lie the gills. 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. 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. [Illustration: FIG. 3--THIRD MAXILLIPED OF LOBSTER. (From British Museum Guide.)]