S D

Nematoda 0 Nemertea 0 EchiuraI

Mesozoic

Entoprocta I

Cenozoic

Rotifera Nematomorpha

Placozoa "Mesozoa" Platyhelminthes Gnathostomulida Gastrotricha Acanthocephala Loricifera Kinorhyncha Pogonophora Sipuncula Phoronida Urochordata

Figure 10.4 Appearance of the main animal phyla and some other high-level taxonomic groups. Geological period abbreviations are standard, ranging from Cambrian (C) to Cretaceous (K). (Based on Valentine 2004.)

Invertebrate body and skeletal plans_

Life on our planet has been evolving for nearly 4 billion years. Molecular data suggest meta-zoans have probably been around for at least at 550 myr, during which time, according to some biologists, as many as 35 separate phyla have evolved. In recent years, new molecular phylogenies have completely changed our views of animal relationships and thus the importance of invertebrate body and skeletal plans. They are important from a functional point of view, but are potentially highly misleading if simply read as telling an evolutionary story. Despite the infinite theoretical possibilities for invertebrate body plans, relatively few basic types have actually become established and many had evolved by the Cambrian (Fig. 10.4). These body plans are usually defined by the number and type of enveloping walls of tissue together with the presence or absence of a celom (Fig. 10.5). The basic unicellular grade is typical of protist organisms and is ancestral to the entire animal kingdom. The first metazoans were multicel-lular with one main cell type and peripheral collar cells or choanocytes, equipped with a whip or flagellum (Nielsen 2008). There are three main body plans (Table 10.1).

The parazoan body plan, seen in sponges, is characterized by groups of cells usually organized in two layers separated by jelly-like material, punctuated by so-called wandering cells or amoebocytes; the cell aggregates are not differentiated into tissue types or organs. In fact molecular phylogenetic studies have suggested that sponges are paraphyletic (see p. 262) so this is only a grade of organization.

The diploblastic grade or body plan, typical of cnidarians and the ctenophorans, has two layers - an outer ectoderm and an inner endo-derm and epithelia. These two layers are separated by the acellular, gelatinous mesogloea.

The triploblastic body plan, seen in most other animals, has three layers of tissues from the outside in: the ectoderm, mesoderm and endoderm. Superimposed on this body plan is the bilateral symmetry that defines the bilate-

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