The next class in the type of Articulates is that of Crustacea,
including Lobsters, Crabs, and Shrimps. It may seem at first that
nothing can be more unlike a Worm than a Lobster; but a comparison of
the class-characters shows that the same general plan controls the
organization in both. The body of the Lobster is divided into a
succession of joints or rings, like that of the Worm; and the fact that
the front rings in the Lobster are soldered together, so as to make a
stiff front region of the body, inclosing the head and chest, while only
the hind rings remain movable, thus forming a flexible tail, does not
alter in the least the general structure, which consists in both of
a body built of articulated rings. The nervous swellings, which were
evenly distributed through the whole body in the Worm, are more
concentrated here, in accordance with the prevalent combination of the
rings in two distinct regions of the body, the larger ones corresponding
to the more important organs; but their relation to the rest of the
organization, and their connection by nervous threads with each other,
remain the same. The respiratory organs, which in most of the Worms were
mere vesicles on the lower part of the sides of the body, are here more
highly organized gills; but their general character and relation to
other parts of the structure are unchanged, and in this respect
the connection of the gills of Crustacea with their legs is quite
significant.
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