The protostomes are arguably the most successful group of animals on Earth. They are the most diverse group, with more than one million species described for arthropods alone. Also, the protostomes contain the nematodes, which probably make up the most abundant and universally distributed of all animal groups.
The protostomes can be divided into two major clades—the lophotrochozoans (including bryozoans, annelids, and mollusks) and the ecdysozoans (including nematodes and arthropods)—largely on the basis of DNA sequence analysis.
The protostomes and deuterostomes are characterized by bilateral symmetry. Hence, these animals are called bilaterians. A bilaterally symmetrical animal can be divided into mirror-image halves by a single plane that passes through the midline of its body.
Bilateral symmetry is characteristic of animals that have a distinct front end, with a head concentrated with sensory and nervous tissues. With this arrangement, the head end typically precedes the rest of the body as the animal moves, allowing an animal to assess a new environment before entering.
The protostomes and other bilatarians also distinguish themselves by the presence of three embryonic cell layers. That is, they are triploblastic, having ectoderm, endoderm, and mesoderm. Mesoderm is the new cell layer found in triploblasts. In these animals, mesoderm has developed into muscles and many other organs and organ systems, which provide their owners with numerous evolutionary advantages.
Protostomes and deuterostomes are clades that have been diversifying separately for more than 500 million years, ever since the Cambrian explosion. The clades are distinguished in part by their early embryology. During gastrulation of these animals, a hollow ball that is one cell thick indents into a cup-shaped structure, forming a blastopore. In protostomes, which is Greek for "mouth first," the mouth arises from the blastopore. In deuterostomes, which means "mouth second," the blastopore becomes the anus, and the mouth forms later.
Let's now focus on the protostomes. The protostomes themselves are extremely varied, but they all have a few features in common. They are bilaterally symmetrical animals whose bodies exhibit two major derived traits: an anterior brain that surrounds the entrance of the digestive tract; and a ventral nervous system consisting of paired or fused longitudinal nerve cords.
Other aspects of the protostome body organization differ widely from group to group. However, the protostomes can be divided into two major clades—the lophotrochozoans and the ecdysozoans—based largely on DNA sequence analysis. The precise placement of a small group, called arrow worms, within the protostomes is still in question. Of the two large groups, the lophotrochozoans include animals that don't undergo molting during their growth, whereas the ecdysozoans do molt. Molting is the shedding of the body's external covering, the cuticle.
Lophotrochozoans were grouped together based on similarities in genomic sequences. Many lineages, with the notable exception of mollusks, have a wormlike body form, which enables them to burrow efficiently through marine sediment or soil. The name lophotrochozoan comes from two different ciliated features. One structure, called a lophophore, is a circular or U-shaped ring of ciliated, hollow tentacles around the mouth that is used for both food collection and gas exchange. The lophophore appears to have evolved independently at least twice, or else it is an ancestral feature of lophotrochozoans and has been lost in many groups.
The other notable structure of lophotrochozoans is a free-living larval form known as a trochophore. Neither the lophophore nor the trochophore is universal to all lophotrochozoans, however.
Of lophotrochozoans, annelids deserve mention because they have evolved a useful adaptation: segmentation. Each segment is separated by a septum that isolates it from the adjacent segment. Segmentation allows an animal to move different parts of its body independently, giving it much better control of its movement.
The other major clade of protostomes, the ecdysozoans, is characterized by having an external covering, or cuticle, that is secreted by the underlying epidermis (the outermost cell layer). The cuticle provides these animals with both protection and support. Once formed, however, the cuticle cannot grow. These animals increase in size by shedding, or molting, the cuticle and replacing it with a new, larger one. This molting process gives the clade its name. In Greek, ecdysis means "to get out of."
A fossil Cambrian arthropod preserved in the process of molting shows that molting evolved more than 500 million years ago. An increasingly rich array of molecular and genetic evidence, including a set of Hox genes shared by all ecdysozoans, suggests the ecdysozoans have a single common ancestor. Thus, molting of a cuticle is a trait that may have evolved only once during animal evolution.
In many ecdysozoans that have wormlike bodies, the cuticle is relatively thin and flexible; it offers the animal some protection but provides only modest body support. A thin cuticle allows the exchange of gases, minerals, and water across the body surface, but it restricts the animal to moist habitats.
The cuticles of other ecdysozoans, mainly arthropods, function as external skeletons, or exoskeletons. These exoskeletons are thickened by layers of protein and a strong, waterproof polysaccharide called chitin. Predation was a major selection pressure in evolution that favored the development of hard external body coverings. Like annelids in the lophotrochozoan clade, arthropod bodies are segmented. The segments typically differ strikingly from one another and bear specialized appendages.
The appendages are manipulated by muscles, which are attached to the inside of the exoskeleton. Each segment has muscles that operate that segment and the appendages attached to it. The jointed appendages of arthropods give the clade its name, from the Greek words arthron, which means "joint," and podos, which means "foot" or "limb."
In this body plan of a crustacean, which is an arthropod, appendages have become specialized for sensing, holding food, walking and gathering food, and swimming.
Jointed, specialized appendages permit complex patterns of movement, including, in insects, the ability to fly. With flight, insects took advantage of new feeding and lifestyle opportunities. These opportunities, in turn, led to the unparalleled evolutionary success of insects.
Protostomes are a group of animals with several shared derived traits, but otherwise differ from each other radically in body form. The shared derived traits of protostomes include a blastopore that develops into a mouth, an anterior brain, and a ventral nervous system.
The protostomes are divided into two major clades, the lophotrochozoans and the ecdysozoans. Annelids and mollusks, among other groups, are members of the lophotrochozoans. Several lophotrochozoan groups are characterized by a filter-feeding structure known as a lophophore or by cilia-bearing larvae known as trochophores.
Arthropods and nematodes are members of the ecdysozoans. Ecdysozoans, which have a body covering known as a cuticle, must molt periodically in order to grow.