The large number and sheer diversity of protostomes necessitates a restriction on the kinds of behaviors (and species) that can be discussed in this chapter. The behaviors highlighted here are based in part on their importance to the survival of an individual organism.
Protostomes are some of the most morphologically complex, ecologically diverse, and behaviorally versatile organisms in the animal kingdom. They consist of more than one million species divided into approximately 20 phyla. Major representative phyla include the Platyhelminthes (flukes, pla-narians, and tapeworms), Nematoda (roundworms), Mollusca (chitons, clams, mussels, nautiluses, octopods, oysters, snails, slugs, squids, and tusk shells), Annelida (bristleworms, earthworms, leeches, sandworms, and tubeworms), and Arthro-poda (ants, centipedes, cockroaches, crabs, crayfish, lobsters, millipedes, scorpions, spiders, and ticks).
When considering what a protostome is, it is important to note that the answer appears to be changing with the accumulation of new information. Evidence from studies of morphology and the fossil record generally support the view that animals in the phyla Annelida, Mollusca, and Arthropoda are indeed protostomes. However, new data based on rRNA analysis suggests that some animals in the Pseudocoelomate phyla (gastrotrichs, rotifers, and roundworms) and in the Acoelomate phyla (flukes, planarians, tapeworms, and ciliated worms) are also protostomes.
Despite the impressive diversity of organisms in this group, the vast majority of protostomes share certain basic characteristics of embryonic development. Indeed, the very name protostomes means "first mouth" and nicely illustrates the common characteristic that the initial opening to the digestive tract in the embryo develops into a mouth. Additional protostome characteristics include an embryonic stage known in the literature as mosaic development. Mosaic development produces a series of cell divisions (cleavage patterns) in which the fate of individual cells following the first cell division is fixed (determinate cleavage), while subsequent cell divisions are arranged spirally (spiral cleavage). Moreover, in proto-stomes the origin of the mesoderm (the germ layer producing such structures as the heart, muscles, and circulatory organs) is created from both the ectoderm (the germ layer producing the skin or integument, nervous system, mouth and anal canals) and endoderm (the germ layer producing the linings of the digestive tract and related glands) in a region known as the 4d cell. Protostomes also have an internal body cavity situated between the digestive tract and the body wall known as the coelom. Two cylindrical masses constructed from mesodermal cells split and the resulting cavities enlarge and combine to form an internal body cavity (coelom) that is surrounded on all sides by mesoderm cells (schizocely).
All protostomes must engage in activities that lead to survival and reproduction. The honey bee and ant, for example, must find and digest food and protect the colony. The pla-narian and crab must also meet nutritional requirements, reproduce, and defend themselves, but do so often in an aquatic environment. An earthworm is faced with similar problems of survival, but usually solves them underground. Protostomes that fly, swim, or burrow are all faced with the same set of problems. The solutions to these problems represent an interaction of environment and morphology, and here lies the differences in what is called behavior.
The word "behavior" is ambiguous. A physiologist, for example, may be comfortable describing the "behavior of a neuron," while a behavioral scientist might find this objectionable. Moreover, among behavioral scientists there are often discrepancies in the definition of behavior. John B. Watson, who popularized an early form of "behaviorism" in the early twentieth century, once defined behavior as muscle contractions and glandular secretions. Other behavioral scientists such as B. F. Skinner have used several definitions of behavior, including "the movement of an organism in space in relation either to its point of origin or to some other object." Many of these definitions give a novice the impression that, for behavioral scientists, the subject matter consists of bodily movements and mechanical responses. Many define behavior not as movement of an organism (which is the proper study of kinesiology), but as an act. Defining behavior in terms of actions captures the notion that behavior has consequences, in other words, scientists are primarily interested in what an organism "does." By defining behavior in terms of actions and consequences, the focus of a behavioral analysis is not on the individual movements that constitute a behavior (as important as this is), but what the behavior "accomplishes." For instance, how an organism acts in a social situation, responds to threats, or captures food is
intimately related to its body plan. Protostomes have a symmetrical body plan (e.g., planarians, earthworms, lobsters, or ants). One of the more interesting body plans is radial symmetry. Animals with radial symmetry have no front or back and take the general form of a cylinder (e.g., sea stars, and sea anemones) with various body parts connected to a main axis. Such animals have feeding structures and sensory systems that interact with their environment in all directions. Such a body plan is most common among animals that are permanently attached to a substrate (e.g., sea anemones) or drifting in the open seas (e.g., jellyfishes).
Another type of symmetrical body plan found in proto-stomes is bilateral symmetry. Invertebrates with bilateral symmetry (e.g., planarians, earthworms, crustaceans, insects, and spiders) have a definite front and back, left and right, and backside and underside orientation. Animals with such a body plan generally can control their locomotion, unlike sessile or drifting species (radial symmetry). The front end (anterior) contains an assortment of feeding and sensory structures, often encapsulated in a head (cephalization) that confronts the environment first. Moreover, the underside (ventral surface) typically contains structures necessary for locomotion, and the backside (dorsal) becomes specialized for protection.
Feeding behavior consists of several different types of acts associated with discovery, palatability, and ingestion. The expression of feeding behavior is a combination of evolutionary and environmental pressures. Depending on the species, pro-tostomes consume an infinite variety of food ranging from microscopic organisms, vegetable matter, and other protostomes; some even grow their own food. Despite the large and varied number of protostomes, some generalizations can be found. First, the strategies for finding food can be reduced to those organisms that find food by living on it, foraging for it, waiting for it to pass by, growing it, and having other organisms provide it. Second, the mechanisms associated with feeding can be reduced to those that singly and/or in combination feed by suspension, deposition, macroherbivory and predation.
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