Chapter 32 Biology

diploblastic.

2 germ layers

eumetazoans

Member of a clade of animals with true tissues.

larva

a sexually immature form of an animal that is morphologically distinct from the adult, usually eats different food, and may even have a different habitat than the adult, as in the case of the aquatic larva of a mosquito or dragonfly

Ventral

bottom (belly side)

Ecdysozoa

-Supergroup of protostomes; characterized by periodic molting of their exoskeleton. Include the roundworms and arthropods. -secrete external skeletons (exoskeletons); the stiff covering of a cricket and the flexible cuticle of a nematode are examples -refers to a characteristic shared by nematodes, arthropods, and some of the other ecdysozoan phyla

body cavity

A fluid-containing space between the digestive tract and the body wall.

Are acoelomate flatworms basal bilaterians?

A series of recent molecular papers have indicated that acoelomate flatworms (phylum Acoela) are basal bilaterians. A different conclusion was supported by a 2011 analysis, which placed acoelomates within Deuterostomia. Researchers are currently sequencing the genomes of several acoelomates and species from closely related groups to provide a more definitive test of the hypothesis that acoelomate flatworms are basal bilaterians. If further evidence supports this hypothesis, this would suggest that the bilaterians may have descended from a common ancestor that resembled living acoelomate flatworms—that is, from an ancestor that had a simple nervous system, a saclike gut with a single opening (the "mouth"), and no excretory system.

Compare three aspects of the early development of a snail (a mollusc) and a human (a chordate).

A snail has a spiral and determinate cleavage pattern; a human has radial, indeterminate cleavage. In a snail, the coelomic cavity is formed by splitting of mesoderm masses; in a human, the coelom forms from folds of archenteron. In a snail, the mouth forms from the blastopore; in a human, the anus develops from the blastopore.

determinate cleavage

A type of embryonic development in protostomes that rigidly casts the developmental fate of each embryonic cell very early.

Sponges are basal animals

Among the extant taxa, sponges (phylum Porifera) branch from the base of the animal tree. Recent morphological and molecular analyses indicate that sponges are monophyletic

pseudocoelomates

An animal whose body cavity is lined by tissue derived from mesoderm and endoderm. -it is a fully functional body cavity

Ediacaran biota

An early group of soft-bodied, multicellular eukaryotes known from fossils that range in age from 565 million to 545 million years old.

gastrula

An embryonic stage in animal development encompassing the formation of three layers: ectoderm, mesoderm, and endoderm.

Based on the tree in Figure 32.11, which statement is false? a. The animal kingdom is monophyletic. b. Acoelomate flatworms are more closely related to echinoderms than to annelids. c. Sponges are basal animals. d. Bilaterians form a clade

B

Most of the fossils from the Cambrian explosion are

Bilatarian

Most animal phyla belong to the clade Bilateria.

Bilateral symmetry and the presence of three prominent germ layers are shared derived characters that help define the clade Bilateria. This clade contains the majority of animal phyla, and its members are known as bilaterians. The Cambrian explosion was primarily a rapid diversification of bilaterians.

There are three major clades of bilaterian animals.

Bilaterians have diversified into three main lineages, Deuterostomia, Lophotrochozoa, and Ecdysozoa. With one exception, the phyla in these clades consist entirely of invertebrates, animals that lack a backbone; Chordata is the only phylum that includes vertebrates, animals with a backbone.

bilateral symmetry

Body plan in which only a single, imaginary line can divide the body into two equal halves. -has two axes of orientation: front to back and top to bottom. -have dorsal, ventral, anterior, and posterior sides

Describe how body plans provide useful information yet should be interpreted cautiously as evidence of evolutionary relationships.

Body plans provide a helpful way to compare and contrast key features of organisms. However, phylogenetic analyses show that similar body plans have arisen independently in different groups of organisms. As such, similar body plans may have arisen by convergent evolution and hence may not be informative about evolutionary relationships

Which of the following was probably the least important factor in bringing about the Cambrian explosion? a. the emergence of predator-prey relationships b. an increase in the concentration of atmospheric oxygen c. the movement of animals onto land d. the origin of Hox genes

C

Describe the evidence that cnidarians share a more recent common ancestor with other animals than with sponges

Cnidarians possess true tissues, while sponges do not. Also unlike sponges, cnidarians exhibit body symmetry, though it is radial and not bilateral as in other animal phyla

All animals share a common ancestor

Current evidence indicates that animals are monophyletic, forming a clade called Metazoa. All extant and extinct animal lineages have descended from a common ancestor

What caused the Cambrian explosion? Describe current hypotheses

Current hypotheses about the cause of the Cambrian explosion include new predator-prey relationships, an increase in atmospheric oxygen, and an increase in developmental flexibility provided by the origin of Hox genes and other genetic changes.

The distinction between sponges and other animal phyla is based mainly on the absence versus the presence of a. a body cavity. b. a complete digestive tract c. mesoderm. d. true tissues

D

Distinguish the terms grade and clade.

Grade-level characteristics are those that multiple lineages share regardless of evolutionary history. Some grade-level characteristics may have evolved multiple times independently. Features that unite clades are derived characteristics that originated in a common ancestor and were passed on to the various descendants.

deuterostome development

In animals, a developmental mode distinguished by the development of the anus from the blastopore; often also characterized by radial cleavage and by the body cavity forming as outpockets of mesodermal tissue. -most have indeterminate cleavage

protostome development

In animals, a developmental mode distinguished by the development of the mouth from the blastopore; often also characterized by spiral cleavage and by the body cavity forming when solid masses of mesoderm split. *some have determinate cleavage

Evaluate whether the origin of cell-to-cell attachment proteins in animals illustrates descent with modification. (See Concept 22.2.)

In descent with modification, an organism shares characteristics with its ancestors (due to their shared ancestry), yet it also differs from its ancestors (because organisms accumulate differences over time as they adapt to their surroundings). As an example, consider the evolution of animal cadherin proteins, a key step in the origin of multicellular animals. These proteins illustrate both of these aspects of descent with modification: Animal cadherin proteins share many protein domains with a cadherin-like protein found in their choanoflagellate ancestors, yet they also have a unique "CCD" domain that is not found in choanoflagellates.

Summarize the main stages of animal development. What family of control genes plays a major role?

In most animals, the zygote undergoes cleavage, which leads to the formation of a blastula. Next, in gastrulation, one end of the embryo folds inward, producing layers of embryonic tissue. As the cells of these layers differentiate, a wide variety of animal forms are produced. Despite the diversity of animal forms, animal development is controlled by a similar set of Hox genes across a broad range of taxa.

Consider clades Bilateria, Lophotrochozoa, Metazoa, Chordata, Ecdysozoa, Eumetazoa, and Deuterostomia. List the clades to which humans belong in order from the most to the least inclusive clade.

Listed in order from the most to the least inclusive clade, humans belong to Metazoa, Eumetazoa, Bilateria, Deuterostomia, and Chordata.

Are ctenophores basal metazoans?

Many researchers have concluded that sponges are basal metazoans. However, several recent studies have placed the comb jellies (phylum Ctenophora) at the base of the animal tree. Data that are consistent with placing sponges at the base of the animal tree include fossil steroid evidence, molecular clock analyses, the morphological similarity of sponge collar cells to the cells of choanoflagellates, and the fact that sponges are one of the few animal groups that lack true tissues (as might be expected for basal animals). Ctenophores, on the other hand, have true tissues and their cells do not resemble the cells of choanoflagellates. At present, the idea that ctenophores are basal metazoans remains an intriguing but controversial hypothesis.

Evaluate this claim: Ignoring the details of their specific anatomy, worms, humans, and most other triploblasts have a shape analogous to that of a doughnut.

Most coelomate triploblasts have two openings to their digestive tract, a mouth and an anus. As such, their bodies have a structure that is analogous to that of a doughnut: The digestive tract (the hole of the doughnut) runs from the mouth to the anus and is surrounded by various tissues (the solid part of the doughnut). The doughnut analogy is most obvious at early stages of development (see Figure 32.10c).

radial cleavage

The cleavage planes are either parallel or perpendicular to the vertical axis of the embryo; as seen at the eight-cell stage, the tiers of cells are aligned, one directly above the other

What animal characteristics would be needed by an imaginary plant that could chase, capture, and digest its prey—yet could also extract nutrients from soil and conduct photosynthesis?

The imaginary plant would require tissues composed of cells that were analogous to the muscle and nerve cells found in animals: "Muscle" tissue would be necessary for the plant to chase prey, and "nerve" tissue would be required for the plant to coordinate its movements when chasing prey. To digest captured prey, the plant would need to either secrete enzymes into one or more digestive cavities (which could be modified leaves, as in a Venus flytrap) or secrete enzymes outside of its body and feed by absorption. To extract nutrients from the soil—yet be able to chase prey—the plant would need something other than fixed roots, perhaps retractable "roots" or a way to ingest soil. To conduct photosynthesis, the plant would require chloroplasts. Overall, such an imaginary plant would be very similar to an animal that had chloroplasts and retractable roots

Are sponges monophyletic?

Traditionally, sponges were placed in a single phylum, Porifera. This view began to change in the 1990s, when molecular studies indicated that sponges were paraphyletic; as a result, sponges were placed into several different phyla that branched near the base of the animal tree. Since 2009, however, several morphological and molecular studies have concluded that sponges are a monophyletic group after all, as traditionally thought. Researchers are currently sequencing the entire genomes of various sponges to investigate whether sponges are indeed monophyletic.

Describe key ways that animals differ from plants and fungi.

Unlike animals, which are heterotrophs that ingest their food, plants are autotrophs, and fungi are heterotrophs that grow on their food and feed by absorption. Animals lack cell walls, which are found in both plants and fungi. Animals also have muscle tissue and nerve tissue, which are not found in either plants or fungi. In addition, the sperm and egg cells of animals are produced by meiotic division, unlike what occurs in plants and fungi (where reproductive cells such as sperm and eggs are produced by mitotic division). Finally, animals regulate the development of body form with Hox genes, a unique group of genes that is not found in either plants or fungi.

Suppose the most recent common ancestor of extant fungi and animals lived 1 billion years ago. If the first fungi lived 990 million years ago, would extant animals also have been alive at that time? Explain

We cannot infer whether extant animals originated before or after extant fungi. If correct, the date provided for the most recent common ancestor of fungi and animals would indicate that animals originated some time within the last billion years. The fossil record indicates that animals originated at least 560 million years ago. Thus, we could conclude only that animals originated sometime between 1 billion years ago and 560 million years ago.

Among the characteristics unique to animals is a. gastrulation. b. multicellularity c. sexual reproduction. d. flagellated sperm.

a

bilaterians

a clade whose members have a complete digestive tract and a bilaterally symmetric form

lophophore

a crown of ciliated tentacles that function in feeding

cleavage

a succession of mitotic cell divisions without cell growth between the divisions

Eumetazoa is a clade of animals with true tissues.

all animals except for sponges and a few other groups belong to a clade of eumetazoans (true animals). True tissues evolved in the common ancestor of living eumetazoans. Basal eumetazoans, which include the phyla Ctenophora (comb jellies) and Cnidaria, are diploblastic and generally have radial symmetry.

Acoelomate

an animal that lacks a coelom, or body cavity

coelomates

animals that possess a true coelom

During the development of most animals, cleavage leads to the formation of a multicellular stage called

blastula

radial symmetry

body plan in which body parts repeat around the center of the body (each slice of pie similar)

Put the following milestones in animal evolution in order from oldest to most recent: (a) origin of mammals, (b) earliest evidence of terrestrial arthropods, (c) Ediacaran fauna, (d) extinction of large, nonflying dinosaurs.

c, b, a, d

Lophotrochozoa

clade of protostomes that exhibit a trochophore larvae stage or a lophophore feeding structure -two different features observed in some animals belonging to this clade. Some lophotrochozoans, such as ectoprocts, develop a unique structure called a lophophore

This body cavity is also called a

coelom

Indeterminate cleavage

each cell produced by early cleavage divisions retains the capacity to develop into a complete embryo. For example, if the cells of a sea urchin embryo are separated at the four-cell stage, each can form a complete larva. Similarly, it is the indeterminate cleavage of the human zygote that makes identical twins possible.

archenteron

embryo's developing digestive tube initially forms as a blind pouch (ancherteron) -becomes the gut -As the archenteron forms in protostome development, initially solid masses of mesoderm split and form the coelom. -In deuterostome development, the mesoderm buds from the wall of the archenteron, and its cavity becomes the coelom.

mesoderm

fills much of the space between the ectoderm and endoderm

Following the blastula stage is the process of ___, during which the layers of embryonic tissues that will develop into adult body parts are produced

gastrulation

Ectoderm

germ layer covering the surface of the embryo, gives rise to the outer covering of the animal and, in some phyla, to the central nervous system

tissues

groups of similar cells that act as a functional unit

triploblastic

having three germ layers -In triploblasts, the mesoderm forms the muscles and most other organs between the digestive tract and the outer covering of the animal

blastula

in many animals takes the form of a hollow ball

All bilaterally symmetrical animals have a third germ layer, called the

mesoderm

a "true" coelom forms from tissue derived from

mesoderm -The inner and outer layers of tissue that surround the cavity connect and form structures that suspend the internal organs

Animal larvae eventually undergo ___________, a developmental transformation that turns the animal into a juvenile that resembles an adult but is not yet sexually mature.

metamorphosis

Deuterostomia

one of the three main lineages of bilaterian animals -hemichordates (acorn worms), echinoderms (sea stars and relatives), and chordates are members of the bilaterian clade Deuterostomia

Cambrian explosion

rapid diversification of most major animal groups marking the start of the Paleozoic era A burst of evolutionary origins when most of the major body plans of animals appeared in a relatively brief time in geologic history; recorded in the fossil record about 545 to 525 million years ago.

Endoderm

the innermost germ layer, lines the pouch that forms during gastrulation (the archenteron) and gives rise to the lining of the digestive tract (or cavity) and organs such as the liver and lungs of vertebrates

blastopore

the opening of the central cavity of an embryo in the early stage of development. -the indentation that during gastrulation leads to the formation of the archenteron. After the archenteron develops, in most animals a second opening forms at the opposite end of the gastrula

spiral cleavage

the planes of cell division are diagonal to the vertical axis of the embryo; as seen in the eight-cell stage of the embryo, smaller cells are centered over the grooves between larger, underlying cells

Dorsal

top side (back side)

Posterior

toward the back of the body

Anterior

toward the front

Because of the presence of a mesoderm, bilaterally symmetrical animals are considered:

triploblastic

Individuals in other phyla, including molluscs and annelids, go through a distinctive developmental stage called the

trochophore larva