Arthropod Evolution

2.2. Trilobita

The trilobites (Figure 1.2), of which almost 4000 species have been described, are marine fossils that reached their peak diversity in the Cambrian and Ordovician periods (500-600 million years ago) (Whittington, 1992). Despite their antiquity they were, however, not primitive but highly specialized arthropods. In contrast to modern arthropods the trilobites as a whole show a remarkable uniformity ofbody structure. The body, usually

FIGURE 1.2. Triarthrus eatoni (Trilobita). (A) Dorsal view; and (B) ventral view. [From R. D. Barnes, 1968, Invertebrate Zoology, 2nd ed. By permission of the W. B. Saunders Co., Philadelphia.]

oval and dorsoventrally flattened, is divided transversely into three tagmata (head, thorax, and pygidium) and longitudinally into three lobes (two lateral pleura and a median axis). The head, which bears a pair of antennae, compound eyes, and four pairs of biramous appendages, is covered by a carapace. A pair of identical biramous appendages is found on each thoracic segment. The basal segment of each limb bears a small, inwardly projecting endite that is used to direct food toward the mouth.

Much about the habits of trilobites can be surmised from examination of their remains and the deposits in which these are found. Most trilobites lived near or on the sea floor. While some species preyed upon small, soft-bodied animals, the majority were scavengers. However, like earthworms, a few smaller trilobites took in mud and digested the organic matter from it. On the basis of X-ray studies of pyritized trilobite specimens, which show that trilobites possess a combination of chelicerate and crustacean characteristics, Cisne (1974) concluded that the Trilobita, Chelicerata, and Crustacea form a natural group with a common ancestry. Their ancestor would have a body form similar to that of trilobites. Most authors dispute the proposed trilobite-crustacean link, and some even reject the association between trilobites and chelicerates. Indeed, there are those who suggest that the trilobites themselves are polyphyletic (Willmer, 1990).

Although the decline of trilobites (and their replacement by the crustaceans as the dominant aquatic arthropods) is a matter solely for speculation, Tiegs and Manton (1958) suggested that their basic, rather cumbersome body plan may have prohibited the evolution of fast movement at a time when highly motile predators such as fish and cephalopods were becoming common. In addition, the many identical limbs presumably moved in a metachronal manner, which is a rather inefficient method in large organisms.

2.3. The Chelicerate Arthropods

The next four groups are often placed together under the general heading of Chelicerata because their members possess a body that is divisible into cephalothorax and abdomen, the former usually bearing a pair of chelicerae (but lacking antennae), a pair of pedipalps, and four pairs of walking legs. Although there is little doubt of the close relationship between the Xiphosura, Eurypterida, and Arachnida, the position of the Pycnogonida is uncertain. Though they are usually included as a class of chelicerates, their affinities with other members of this group remain unclear, and there are some authors who consider they deserve more separated status (see King, 1973; Manton, 1978; Edgecombe, 1998; Fortey and Thomas, 1998).

Xiphosura. Limulus polyphemus, the king or horseshoe crab (Figure 1.3), is one of four surviving species of a class of arthropods that flourished in the Ordovician-Upper Devonian periods. King crabs occur in shallow water along the eastern coasts of North and Central America. Three species of Tachypleus and Carcinoscorpius occur along the coasts of China, Japan, and the East Indies. Like trilobites they are bottom feeders, stirring up the substrate and extracting the organic material from it. In Limulus the cephalothorax is covered with a horseshoe-shaped carapace. The abdomen articulates freely with the cephalothorax and at its posterior end carries a long telson. On the ventral side of the cephalothorax are six pairs of limbs. The most anterior pair are the chelicerae, and these are followed by five pairs of legs. Each leg has a large gnathobase, which serves to break up food and pass it forward to the mouth. Six pairs of appendages are found on the abdomen. The first pair fuse medially to form the operculum. This protects the remaining pairs, which bear gills on their posterior surface.

FIGURE 1.3. The horseshoe crab, Limulus polyphemus. (A) Dorsal view and (B) ventral view. [From R. D. Barnes, 1968, Invertebrate Zoology, 2nd ed. By permission of the W. B. Saunders Co., Philadelphia.]

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

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