Important words and concepts from Chapter 32, Campbell & Reece, 2002 (3/25/2005):

by Stephen T. Abedon ( for Biology 113 at the Ohio State University



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(1) Chapter title: Introduction to Animal Evolution

(a)                    [introduction to animal evolution (Google Search)] [index]

(2) Animals

(a)                    Relationships between animal phyla tend to be determined by comparing characteristics of extant animals, rather than from the fossil record; this is because the fossil record, for the diversification of animals into existing phyla, is incomplete (probably because most of this diversification occurred prior to the invention of hard parts)

(b)                    Animals probably represent a monophyletic taxon

(c)                    Animals…

(i)                      are multicellular chemoheterotrophs

(ii)                    mostly eat other organisms or eat the remains of other organisms

(iii)                   mostly consume by ingesting (taking food into their bodies)

·        Note that this form of acquisition of nourishment contrasts with that of the fungi which, instead, live within their food

·        However, otherwise the animals and the fungi are similar in that both secrete hydrolytic enzymes into their food, then absorb the resulting digestate

(iv)                  store energy as glycogen

(v)                    lack cell walls

(vi)                  employ collagen as a structural protein (holds tissues/bodies together)

(vii)                 possess nervous tissue and muscle tissue

(viii)               are diploid

(ix)                  reproduce sexually

(x)                    employ motile sperm and non-motile eggs

(xi)                  typically develop from embryos (as also do plants)

(d)                    [early animal evolution (Google Search)] [images of animals (1)] [images of animals (2)] [invertebrate paleontology image gallery (Peabody Museum of Natural History – Yale University)] [index]




(3) Invertebrates

(a)                    Most animal phyla are invertebrates (in fact, all but one animal phylum contain nothing but invertebrates)

(b)                    Extant animals are grouped into approximately 35 phyla

(c)                    In our survey of the invertebrates (chapter 33) we will consider only 10 of these













(d)                    These phyla may be grouped according to their adult and embryological forms into

(i)                      Parazoa vs. Eumetazoa

(ii)                    Radiata vs. Bilateria

(iii)                   Diploblastic vs. Triploblastic

(iv)                  Acoelomates vs. Pseudocoelomates vs. Coelomates

(v)                    Protostomes vs. Deuterostomes

(e)                    We will consider these terms, and others, before distinguishing phyla

(f)                     See Figure 32.4, A traditional view of animal diversity based on body-plan grades for an overview of invertebrate relationships and characteristics

(g)                    [invertebrates (Google Search)] [invertebrates and animal diversity (good online outline of invertebrate diversity with brief descriptions of phyla members though no better than what is in your text and is based on old text edition) (Biology 201 – George C. Brown)] [index]




(4) Parazoa versus Eumetazoa

(a)                    Animals presumably evolved from unicellular eukaryotes, i.e., protozoa

(b)                    A very early split among animal types was between the parazoa and the eumetazoa, animals lacking and animals possessing true tissues, respectively

(c)                    The only extant Parazoans are members of phylum Porifera

(d)                    See Figure 32.4, A traditional view of animal diversity based on body-plan grades

(e)                    [parazoa, eumetazoa (Google Search)] [index]

(5) Radiata versus Bilateria

(a)                    Also fairly early in the evolution of animals was a split between animals that possess radial symmetry (radiata) and those that possess bilateral symmetry (bilateria)

(b)                    See Figure 32.4, A traditional view of animal diversity based on body-plan grades

(c)                    See Figure 32.5, Body symmetry

(d)                    [(Google Search)] [index]

(6) Radiata

(a)                     Animals with radial symmetry possess a top and a bottom, but no distinct front, back, left, or right

(b)                    "Many radial animals are sessile forms (attached to substratum) or planktonic (drifting or weakly swimming aquatic forms). Their symmetry equips them to meet the environment equally well from all sides. More active animals are generally bilateral."

(c)                    All members of radiata are also diploblastic

(d)                    See Figure 32.4, A traditional view of animal diversity based on body-plan grades

(e)                    See Figure 32.5, Body symmetry

(f)                      [radiata (Google Search)] [index]

(7) Bilateria (dorsal, ventral, anterior, posterior, cephalization)

(a)                    "A bilateral animal has not only a top, or dorsal side, and bottom, or ventral side, but also a head, anterior, end and tail, or posterior, end and a left and right sides."

(b)                    "Associated with bilateral symmetry is cephalization, an evolutionary trend toward the concentration of sensory equipment on the anterior end, the end of a traveling animal that is usually first to encounter food, danger, and other stimuli."

(c)                    Note: "The radial symmetry of some animals apparently evolved secondarily from a bilateral condition as an adaptation to a more sedentary lifestyle." Examples of such organisms can be found among the echinoderms

(d)                    All members of bilateria are also triploblastic

(e)                    See Figure 32.4, A traditional view of animal diversity based on body-plan grades

(f)                      See Figure 32.5, Body symmetry

(g)                    [bilateria, cephalization (Google Search)] [index]

(8) Gastrulation (archenteron, blastopore)

(a)                    Gastrulation is the mechanism during animal development which is responsible for the differentiating of the animal tissues into what we call germinal layers

(b)                    Gastrulation involves an invagination of cells during the blastula stage of development to form a digestive cavity known as the archenteron

(c)                    The single opening to this cavity is the blastopore

(d)                    See Figure 32.1, Early embryonic development

(e)                    See Figure 32.2, One hypothesis for the origins of animals from a flagellated protist

(f)                      [gastrulation, archenteron, blastopore (Google Search)] [index]

(9) Germ layers (germinal layers, endoderm, mesoderm, ectoderm,  diploblastic, triploblastic)

(a)                    The developing animal embryo folds and forms into three layers called germinal (germ) layers, listed from inside going out:

(i)                      Endoderm

(ii)                    Mesoderm

(iii)                   Ectoderm

(b)                    See Figure 32.1, Early animal development

(c)                    Not all animals possess all three germinal layers

(d)                    Those that possess all three layers are termed triploblastic and include phyla

(i)                      Platyhelminthes

(ii)                    Rotifera

(iii)                   Nematoda

(iv)                  (Nemertea)

(v)                    (Bryozoa)

(vi)                  (Phoronida)

(vii)                 (Brachiopoda)

(viii)               Mollusca

(ix)                  Annelida

(x)                    Arthropoda

(xi)                  Echinodermata

(xii)                 Chordata

(e)                    Those that possess only endoderm and ectoderm (the Radiata) are termed diploblastic and include phyla

(i)                      Cnidaria

(ii)                    (Ctenophora)

(f)                      In addition, characteristics of the mesoderm can differ fundamentally between phyla (see body cavities, below)

(g)                    [germ layers, germinal layers, endoderm, mesoderm, ectoderm, diploblastic, triploblastic (Google Search)] [index]

(10) Body cavities (acoelomates, pseudocoelomates, coelomates) tube within a tube

(a)                    In organisms possessing mesodermal tissue (the Bilateria), the mesoderm can interact with the endoderm (which forms the gut) in one of three ways

(i)                      Forming a solid mass spanning the volume between ectodermal and the endodermal tissue (acoelomates) and include phylum

·        Platyhelminthes

(ii)                    Forming a solid mass in contact with the ectodermal layer but not the endodermal layer (pseudocoelomates) and include phyla

·        Rotifera

·        Nematoda

(iii)                   In contact with both the ectodermal and endodermal layers, but possessing a cavity within, one that is completely surrounded by the mesodermal tissue (coelomates) and include phyla

·        (Bryozoa)

·        (Phoronida)

·        (Brachiopoda)

·        Mollusca

·        Annelida

·        Arthropoda

·        Echinodermata

·        Chordata

(b)                    See Figure 32.6, Body plans of the bilateria

(c)                    These latter two possibilities represent the "tube-within-a tube body plans, with a fluid-filled cavity separating the digestive tract from the outer body wall."

(d)                    Note that in addition to this tube-within-a-tube, both the pseudocoelomates and coelomates, but not the acoelomates, possess some form of a vascular system that serves to transport nutrients

(e)                    The existence of a body cavity serves to protect internal organs, as well as streamline them with respect to the rest of the organism, thus allowing the evolution of greater organ complexity

(f)                      [body cavities, acoelomates, pseudocoelomates, coelomates (Google Search)] [index]

(11) Protostomes vs. Deuterostomes (gut)

(a)                    The most primitive of eumetazoa (the radiata and the acoelomates) possess a single opening into their digestive cavity, one that serves as both a mouth and an anus

(b)                    A gut is a digestive system that possesses both a mouth and an anus

(c)                    Guts (or, at least, opening specialization) apparently arose at least twice in animals, in both cases this occurred as a consequence of the formation of a second hole in (or into) the digestive cavity

(d)                    Those organisms for which this new hole serves as the mouth are termed Deuterostomes and include phyla

(i)                      Echinodermata

(ii)                    Chordata

(e)                    Those organisms for which this new hole serves as the anus are termed Protostomes and include phyla

(i)                      (Bryozoa)

(ii)                    (Phoronida)

(iii)                   (Brachiopoda)

(iv)                  Mollusca

(v)                    Annelida

(vi)                  Arthropoda

(f)                     See Figure 32.7, A comparison of early development in protostomes and dueterostomes

(g)                    [protostomes, deuterostomes (Google Search)] [index]




(12) Cambrian explosion

(a)                    Approximately 550 million years ago, over a period of 5 to 10 million years most of the extant animal body plans made their first appearance in the fossil record

(b)                    That is, for almost all of the animals alive today, we can trace their ancestors back to this brief period, but no further

(c)                    Soft-bodied fauna probably dates back as far as 700 million years ago, but are thought to have consisted of animals possessing no more sophistication than the acoelomates

(d)                    A key innovation at the Precambrian-Paleozoic boundary (i.e., ~550 million years ago) was the sudden appearance of hard parts in animals

(e)                    The hard parts may have been a response to greater sophistication among predators (i.e., teeth)

(f)                      The hard parts also may explain the sudden appearance of different forms in the fossil record, i.e., different phyla may have existed prior to this time but are grossly underrepresented in the fossil record due to an absence of hard parts; once they possessed hard parts, however, they could "suddenly" appear in the fossil record, possessing fully formed (and recognizable) body plans precursing those observed today

(g)                    Another striking feature of the Cambrian explosion is that no new animal phyla appear after this period, implying that the Cambrian explosion was unique in its ability to generate new animal body plans; “Some researchers speculate that the phyla became locked into developmental patterns that constrained the evolution enough that no additional phyla evolved after the Cambrian explosion” (p. 596, Campbell et al., 1999) (I am sympathetic to this idea of developmental constraints but feel that it is better understood as this: Perhaps the decline in generation of new phyla is best observed at the level of the simpler animals that gave rise to the newly generated phyla, with those very simple animals either being driven to extinction by their more-sophisticated descendants, or because the more-sophisticated descendants invaded all niches available given slight increases in the sophistication of these very simple animals, thereby closing off adaptive zones whose existence no doubt was essential for driving the development of new and sophisticated developmental strategies and body plans)

(h)                    See Figure 32.13, A sample of some of the animals that evolved during the Cambrian explosion

(i)                      [Cambrian explosion (Google Search)] [index]




(13) Vocabulary [index]

(a)                    Acoelomates

(b)                    Animals

(c)                    Anterior

(d)                    Archenteron

(e)                    Bilateria

(f)                      Blastopore

(g)                    Body cavities

(h)                    Cambrian explosion

(i)                      Ceolomates

(j)                      Cephalization

(k)                    Diploblastic

(l)                      Dorsal

(m)                  Ectoderm

(n)                    Endoderm

(o)                    Gastrulation

(p)                    Germ layers

(q)                    Germinal layers

(r)                     Gut

(s)                     Invertebrates

(t)                      Mesoderm

(u)                    Parazoa versus Eumetazoa

(v)                    Posterior

(w)                  Protostomes vs. Deuterostomes

(x)                    Pseudocoelomates

(y)                    Radiata

(z)                     Radiata versus Bilateria

(aa)                 Triploblastic

(bb)                Ventral