Multicellular, heterotrophic eukaryotes - most obtain nutrients through ingestion (eating)

Lack cell walls - cells bound together by structural proteins (mainly collagen) and intercellular junctions (desmosomes, tight & gap junctions)

unique tissues - muscle & nervous tissue - for movement and conduction of impulses

most reproduce sexually - zygote undergoes cleavage during embryonic development

Cleavage - succession of mitotic cell divisions (of zygote - forms blastula (hollow ball of cells)

Gastrulation - rearrangement of embryo that produces layers of embryonic tissues - blastula invaginates to form gastrula -occurs in all animals except sponges

Tissue on interior of cavity (archenteron) forms endoderm - becomes digestive tract - tissue on exterior forms ectoderm - opening (blastopore) forms either mouth or anus

Appearance of tissues - Parazoa (no true tissue) diverged from Eumetazoa (true tissues)

Appearance of bilateral symmetry - the Radiata (radial symmetry, diploblastic) diverged from the Bilateria (bilateral symmetry, triploblastic)

Body cavities (coelom) - the Acoelomates (no body cavity) and Pseudocoelomates (no mesoderm) diverged from Coelomates (coelom)

Protostomes (coelom from cell masses) diverged from Deuterostomes (coelom from digestive tube)

Group of cells with common structure and function

Members of Parazoa lack true tissues - led to sponges

All other animals have true tissues - collectively known as eumetazoa

Radial - parts of body arranged like spokes on wheel - any imaginary slice through central axis would produce mirror image - members of Radiata characterized by radial symmetry - includes cnidarians and comb jellies

Bilateral - left and right side - only one imaginary slice would divide animal into mirror image halves - members of Bilateria characterized by bilateral symmetry - all other organisms

Radial symmetry

No pronounced cephalization

Diploblastic - have only two germ layers (ectoderm & endoderm)

Bilateral symmetry

Cephalization pronounced - sensory structures concentrated in anterior end

Triploblastic - 3 germ layers (ectoderm, mesoderm & endoderm)

Ectoderm - gives rise to integument (outer covering), central nervous system

Endoderm - gives rise to lining of digestive tube and organs derived from it (e.g., liver & lungs in vertebrates)

Mesoderm - gives rise to muscles and most internal organs in coelom - present in all eumetazoans except members of Radiata (cnidarians & comb jellies)

Coelom - fluid filled cavity (between digestive tract and outer body wall) completely lined with mesoderm

Acoelomates - no coelom - either lack digestive tract or have gastrovascular cavity (single opening) - flatworms

Pseudocoelomates - cavity not lined with mesoderm - rotifers & nematodes

Coelomates - coelom present - all other groups

Protostomes - characterized by spiral & determinate cleavage, schizocoelous development of coelom, and mouth develops from blastopore - see Fig.32.6 (p. 594) in text

Deuterostomes - characterized by radial & indeterminate cleavage, enterocoelous development of coelom, and anus develops from blastopore - see Fig. 32.6 (p. 594) in text

Animals currently grouped into 35 phyla

Most animal phyla appear to have originated in a relatively brief period of time - during late Precambrian and early Cambrian - 545 to 525 million years ago (i.e., the Cambrian explosion)

Studies of developmental genetics suggest that common ancestor of bilateral animals was complex, with cephalization, light detectors, segmentation & nerve cord