Composition of the animal kingdom through geologic time. Only the ptincipal phyla are indicated (names in a box), along with some of the classes they encompass (vertical lines). Related subclasses are connected by an empty box. The time scale indicated was adopted by the Geological Society of London in 1964 (from Kummel 1970). The histogram at the bottom of the figure indicates the percentages of living species, excluding insects (partially from Easton 1960; Shrock and Twenhofel 1953; and from a survey by the author).
say that the early Paleozoic was the age of trilobites, even though their remains dominate the fossil record of that period. We now know that a surprising richness of other marine invertebrates constituted only of soft parts did coexist with the trilobites and that trilobites represented only a small fraction of a major proliferation of arthopods.
The multitude of life forms that existed in the Middle Cambrian period is beautifully illustrated in the book The Burgess Shale by H. Whittington (1985) and more recently traced to the beginning of the Cambrian from the study of the Chengjiang fauna. A view of the latter by a group of collaborating Swedish and Chinese paleontologists (Chen et al. 1991; Xianguang et al. 1991) restricts the development of the principal arthropod groups and of most of the other advanced multicellular life forms to the very transition between the Precambrian and the Cambrian. Undoubtedly, the Cambrian explosion may well have been the most revolutionary and far reaching single event in the history of life. More accessible than their soft-bodied relatives, even if less abundant during their lifetime, the ubiquitous trilobites that we find as fossils provide us all with a telltale reminder of this special time.
Evidently the organizational scheme of the arthopod body, perhaps because of its versatility, has proven most apt to secure survival of some of its offshoots. In its most primitive form, the body plan is modular or metameric, made up of a sequence of basically identical segments (serialhomology) performing similar functions. The latter encompassed locomotion, food gathering, respiration, excretion, and reproduction. The body, made up of a sequence of segments, is generally of elongated form and exhibits bilateral symmetry about an axis carrying the digestive tract. One of the functions of the segments is that of articulating the body. Each segment also usually carries pairs of appendages which are, in turn, divided into jointed segments or podomeres (hence the name Arthropoda, from the Greek arthron = joint, podos = foot). The scheme is highly adaptive. Each block of segments can evolve to respond to specific needs or functions, without limiting the adaptation of other portion of the body to perform different tasks. Such functional groups of segments are called tagmata (from the Greek tagma for a "body of soldiers"). Segmental specialization into tagmata or tagmosis leads to the fairly universal organization of the body into head, thorax, and abdomen. Thus the head tends to monopolize the sensory functions, including eyes, antennae, and related food-gathering and masticatory functions, while successive tagmata may encompass locomotory appendages and/or swimming appendages, etc. The dorsal portion of contiguous plates (tergites) may become fused into a shield (tergum), as that covering the head of most extant and extinct arthropods. In extreme cases, as in the horseshoe crab (Limulus), fusion occurs between head and thorax to form a solid shield called prosoma. Differentiation and specialization can also take place along individual podomeres, giving rise, for example, to antennulae, claws, etc.—the tools that are needed by the tagmata.
The nervous system of the arthropods is highly developed, with pairs of ganglia in most segments, and a pair of nerve cords leading to a more or less developed brain. The interaction of the animal with the environment takes place by means of sensory organs, such as tactile antennulae and single or compound eyes. The latter, as we shall see in detail for the trilobites, can attain surprising complexity and optimization of function. Arthropods possess a circulatory system consisting of heart, arteries, and blood-return ducts. Respiration takes place through gills in the aquatic forms or through a network of tracheae in the air-breathing forms. The sexes are distinct, and eggs hatch either externally or internally. Growth takes many forms. We are familiar with the metamorphosis of the caterpillar into a moth or butterfly.
The soft arthropod body is occasionally encased in a protective shield, or exoskeleton, made mostly of hardened proteins and chitin, which may be further hardened by calcium carbonate or phosphate. The presence of a hard, inextensible exoskeleton poses special problems to the growth of many arthropods. In such cases the increase in size takes place by ecdysis (molting), in which the hard cover is shed periodically, and a new, larger shield is constructed after each
step. Soft-shell crabs are examples of the stage of growth following ecdysis. For a period of time, the animal is protected only by a flexible, proteinaceous exoskeleton that will gradually harden by mineralization.
Based on morphological and anatomical similarities, body segmentation in particular, it has been assumed for a long time that the arthropods may have been related to a common annelid ancestor way back in the Precambrian. During the last twenty years, a new understanding of animal phylogeny has developed, based on the mapping of sequences of nucleotide bases and amino acids for different animal groups. Not only have the arthropods emerged as biochemically different from the annelids, but the reliability of morphological considerations in establishing phylogenetic relationships has been proven misleading in several other cases. Most of the phyla that emerged as a result of the Cambrian explosion are thought to have originated as offshoots of a hypothetical phylum of sluglike animals devoid of true segmentation, called Procoelomata (Bergstrom 1989). Segmentation like that found in arthropods would have been attained from a state of false segmentation, commonly occurring in primitive animals. Several of the offshoots of the Procoelomata are believed to have developed similar morphological characters by convergent evolution.
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