A n annelid's body is made up of a series of repeating units sequentially arranged down the length of the animal. Metamerism describes the process that resulted in the segments, metameres, each of which is identical to the one on either side of it. Arthropods took metamerisation a step farther. Tagmatization grouped adjacent metameres into larger functional units, tagma, responsible for performing specialized tasks. In addition to how the segments were organized into tagma, another important difference between the two phyla is that arthropods have paired appendages on each their metameres.
A word of warning! This sequence of events assumes that arthropods were the logical evolutionary step that followed annelids. There is growing evidence that this may not be the case. This was discussed when we looked at the first animals with a cuticle, the nematodes. If the Ecdysozoa is a valid monophyletic taxon, then this annelid-to-arthropod transition is inappropriate. While we wait for a resolution to this taxonomic conundrum, the transition from a metameric body plan to one based on tagma is probably correct. The question is whether an annelid is the right metameric ancestor.
To understand the impact tagmatization tagmatization has on how an animal functions, let's take a look at the ancestral and modified body plans of two different arthropod subphyla: Crustacea and Uniramia.
Primitive crustaceans had only two tagma, a head and a trunk. The head was formed from five segments and their five pairs of appendages. This included two pairs of sensory appendages: antennae and antennules, and three pairs of feeding appendages: mandibles, first maxillae, and second maxillae used for grinding, cutting, and manipulating food before it was ingested. Each of the trunk appendages had three responsibilities: locomotion, respiration, and food gathering. As this primitive crustacean paddled along using its trunk appendages, it trapped particulate food using setal hairs on the surface and passed it forward to the mouth. While all this was happening, the large surface area of the legs was used for gas exchange. Compare that to the lobster or crayfish. The head tagma has been replaced with a cephalothorax. Some of the trunk appendages have been recruited to help with feeding and maxillipeds are part of the "cephalo" part of the cephalothorax. The "thorax" part, more correctly the pereon with its pereopods, is locomotary and houses the main organ systems. The rest of the trunk, pleon with pleopods, is a muscular tail used to escape from predators and in the females for brooding their eggs.
In the Uniramia centipedes, the head tagma is a fusion of six segments followed by the trunk tagma with a pair of walking legs on each of the segments. How the uniramian head collects and manipulates food before it's ingested hasn't changed much between centipedes and insects. The head also processes sensory information from the antennae and simple eyes. Centipedes have lost the compound eyes. The biggest difference can be seen in changes to the trunk, which in insects is now the thorax and abdomen. This second insect tagma, the thorax, is specialized for locomotion, and the only thing you'll find inside it are muscles that move the wings and legs. The organ systems pass their ducts and tubes through the thorax on their way to the third tagma, the abdomen, which houses the main organ systems. The abdomen's role is the day-to-day functioning of the animal, and it contains the reproductive system.
As the tagma take on these different roles, their shapes, and the shapes of the appendages attached to them, change. Arthropod cuticle is a "living plastic" and can be molded into almost anything you could think of. The result is a tremendous array of different cuticular shapes and forms, many of which are arthropod tools. For example, cuticular appendages can be delicate sensory antennae, shovels, scissors, grinding stones, walking legs, defensive pincers, and paddles for swimming. The possibilities are almost endless, and it's one of the reasons why arthropods are such a successful group.
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