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Spiral cleavage

Spiral cleavage view

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Radial cleavage

Radial cleavage view

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Figure 10.5 Main invertebrate body plans and larvae: upper and lateral views of spiral (a) and radial (b) patterns of cell cleavage; development of the mesoderm in the spiralians (c) and radialians (d); diploblastic (g) and triploblastic (h) body plans and trochophore-type (e) and dipleurula-type (f) larvae.

Figure 10.5 Main invertebrate body plans and larvae: upper and lateral views of spiral (a) and radial (b) patterns of cell cleavage; development of the mesoderm in the spiralians (c) and radialians (d); diploblastic (g) and triploblastic (h) body plans and trochophore-type (e) and dipleurula-type (f) larvae.

Table 10.1 Key characteristics of the three main groups of animals.

Group Grade Symmetry

Key character Larvae

Porifera Parazoan Bilateral and radial symmetry Collar cells Blastula larva

Cnidaria Diploblastic Radial symmetry Cnidoblasts Planula larva

Bilateria Triploblastic Bilateral symmetry Digestive tract Various types rians. And finally the development of the celom or body cavity characterizes most of the animal groups found as fossils. The celom usually functions as a hydrostatic skeleton and is related to locomotion. But the presence and organization of the celom is not phyloge-netically significant; the celom has evolved several times and in some groups, such as the flatworms, there are at least two types of celomic cavities.

The annelid worms and the arthropods have a celom divided along its length into segments; each segment possesses identical paired organs such as kidneys and gonads together with appendages. The mollusks, on the other hand, have an undivided celom situated mesodermally and irregularly duplicated organs.

The remaining bilaterians, such as the pho-ronids, brachiopods, bryozoans, echinoderms and hemichordates have a celom that is divided longitudinally into two or three zones each with different functions. Based around this plan, animals with a specialized feeding and respiratory organ, the lophophore, are characterized by sac-like bodies; but this is no guarantee that these so-called "lophopho-rates", brachiopods and bryozoans, are in fact closely related. The hemichordates possess a crown of tentacles and some have paired gill slits. The echinoderms have an elaborate water vascular system that drives feeding, locomotion and respiration.

The identification of invertebrate body plans is a useful method of grouping organisms according to their basic architecture. However, similarities between grades of construction unfortunately do not always mean a close taxonomic relationship. Be aware that certain body plans have evolved more than once in different groups, Skeletons too, for example, have evolved a number of times in a variety of forms.

The skeleton is an integral part of the body plan of an animal, providing support, protection and attachment for muscles. Many animals such as the soft-bodied mollusks (slugs) possess a hydraulic skeleton in which the movement of fluid provides support. Rigid skeletons based on mineralized material may be external (exoskeleton), in the case of most invertebrates, or internal (endoskeleton) structures, in the case of a few mollusks (e.g. belemnites), echinoderms and vertebrates. Growth is accommodated in a number of ways. Most invertebrate skeletons grow by the addition of new material, a process termed accretion. Arthropods, however, grow by periodic bursts between intervals of ecdysis or molting; echinoderms grow by both accretion to existing material and by the appearance of new calcitic plates.

Classification and relationships_

Classifications based on purely morphological data and embryology have met with problems. Difficulties in establishing homologous characters and homoplasy (see p. 129) have contributed to a number of different phylog-enies. The locator tree (Fig. 10.6), however, outlines some of the main features of animal evolution. From the base of the metazoan tree, the demosponges and calcisponges are the simplest animals whereas the cnidarians are the most basal eumetazoans. Three robust bilaterian groupings are recognized mainly on molecular data: the ecdysozoans, the spira-

lians and the deuterostomes. The ecdysozoans and the spiralians comprise the protostomes ("first mouth") where the mouth develops directly from the first opening, the blastopore, resulting from cell growth and migration. The deuterostomes ("second mouth"), however, have a mouth arising from a secondary opening; the true blastopore often develops as an anus. Not all phyla fit simply into these two major divisions, but using a consensus based on comparative morphology, two main streams emerge: the echinoderm-hemichor-date-chordate (deuterostomous) and the mollusk-lophophorate-annelid-arthropod (protostomous) groupings (Box 10.2).

Other studies have laid emphasis on the similarities between the larval stages of organisms to investigate phylogenetic relationships.

Most invertebrates develop first a larval stage that may be either planktotrophic, free-living and feeding on plankton, or lecithotro-phic, essentially benthic and feeding on yolk sacs. There is a range of different larval types. For example the nauplius larva is most typical of crustaceans, the planula characterizes the cnidarians, the trochophore larva occurs in the mollusks and the polychaetes whereas the shelled veliger also characterizes the mollusks. Thus those groups (annelids and mollusks) with trochophores may have shared a common ancestor. Invertebrate larvae are occasionally identified in the fossil record. With the availability of more advanced preparatory and high-tech investigative techniques, studies of fossil larvae may yet become a viable part of paleontology.

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