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.  The alimentary canal consists of a single digestive cavity passing through the whole body, as in Worms, the anterior part of which is surrounded by a large liver.  What is true of the Lobsters is true also, so far as class-characters are concerned, of all the Crustacea.

Highest in this type are the Insects, and among these I include Spiders and Centipedes as well as Winged Insects.  It is true that the Centipedes have a long uniform body like Worms, and the Spiders have the body divided into two regions like the Crustacea, while the body in true Insects has three distinct regions, head, chest, and hind body; but notwithstanding this difference, both the former share in the peculiar class-character that places them with the Winged Insects in a separate group, distinct from all the other Articulates.  We have seen that in the Worms the respiratory organs are mere vesicles, while in the Crustacea they are more highly organized gills; but in Centipedes, Spiders, and Winged Insects, the breathing-apparatus is aerial, consisting of air-holes on the sides of the body, connected with a system of tubes and vessels extending into the body and admitting air to all parts of it.  In the Winged Insects this system is very elaborate, filling the body with air to such a degree as to render it exceedingly light and adapted to easy and rapid flight.  The general arrangement of parts is the same in this class as in the two others, the typical character being alike in all.