Chapter 33 Review

Acoel flatworms appear to be distinct from other flatworms.

Acoal flatworms, phylum Acoela, were once considered basal to the phylum Platyhelminthes, but they appear to have evolved early in the history of the Bilateria and are the sister taxon to the clade comprised of protostomes and deuterostomes.

Animal Phylogeny

Animals are classified into 35-40 phyla based on shared characteristics.

Current understanding of phylogeny differs from traditional views.

By incorporating molecular data into phylogenetic analyses, some alterations have been made to the traditional view of how members of these phyla are related. Porifera (sponges) constitutes a monophyletic group that shares a common ancestor with other animals. Cnidarians (hydras, sea jellies, and corals) evolved before the origin of bilaterally symmetrical animals. Annelids and arthropods had been considered closely related based on segmentation, but now arthropods are grouped with protostomes that molt their cuticles at least once during their life.

The comb jellies use cilia for movement.

Ctenophora (comb jellies) is a small phylum of medusa-like animals that propel themselves with bands of fused cilia. They may be triploblastic. They capture prey with colloblasts, cells that release an adhesive.

In the rainforest you discover a new species that is terrestrial, has determinate development, molts during its lifetime, and possesses jointed appendages. To which phylum of animals should it be assigned?

Determinate development indicates that it is a protostome and the fact that it molts places it within the Ecdysozoa. The presence of jointed appendages makes it an arthropod.

The evolution of tissues allowed for specialized structures and functions

Each tissue consists of differentiated cells that have characteristic forms and functions.

Animals share some general characteristics

Features common to all animals are multicellularity, heterotrophic lifestyle, and lack of a cell wall. Other features include specialized tissues, ability to move, and sexual reproduction.

Sponges reproduce both asexually and sexually.

Fragments of a sponge are able to grow into complete individuals. Sperm and eggs may be produced by mature individuals; these undergo fertilization to form zygotes that develop into free-swimming larvae that eventually become sessile adults.

Bilaterians have two main types of development.

In a protosome, the mouth develops from or near the blastopore. A protosome has determinate development, and many have spiral cleavage. In a deuterostome, the anus develops from the blastopore. A deuterostome has indeterminate development and radial cleavage.

All cnidarians are carnivores.

Members of the carnivorous and radially symmetrical Cnidaria have distinct tissues but no organs. They are diploblastic and have two body forms: a sessile, cylindrical polyp and a free-floating medusa. Cnidarians are distinguished by capsules called nematocysts that are used in offense and defense. Cnidarians have no circulatory, excretory, or respiratory systems. They have a latticework of nerve cells and are sensitive to touch; some have gravity receptors and light receptors.

A body cavity made the development of advanced organ systems possible.

Most bilaterian animals possess a body cavity other than the gut. A coelom is a cavity that lies within tissues derived from mesoderm. A pseudocoelom lies between tissues derived from mesoderm and the gut (which develops from endoderm). The acoelomate condition and the pseudocoelom appear to have evolved more than once, but the coelom evolved only once. A circulatory system is an example of a specialized organ system that assists with distribution of nutrients and removal of wastes.

Most animals exhibit radial and bilateral symmetry.

Most sponges are asymmetrical, but other animals are bilaterally or radially symmetrical at some time during their life. The body parts of radially symmetrical animals are arranged around a central axis. The body of a bilaterally symmetrical animal has left and right halves. Most bilaterally symmetrical organisms are cephalized and can move directionally.

Parazoa: Animals that lack specialized tissues

Parazoa ("near animals") comprises animals that, for the most part, lack definite symmetry, and that do not possess tissues.

Segmentation allowed for redundant systems and improved locomotion.

Segmentation, which evolved multiple times, allows for efficient and flexible movement because each segment can move somewhat independently. Another advantage to segmentation is redundant organ systems

Protostomes consist of spiralians and ecdysozoans

Spiralia comprises the clades Lophotrochozo and Platyzoa. Spiralian animals grow by gradual addition of mass to the body and undergo spiral cleavage. Ecdysozoans grow by molting the external skeleton; Ecdysozoa includes many varied species, ranging from the pseudocoelomate, unsegmented Nematoda to the coelomate, segmented Arthropoda.

The sponge body is composed of several cell types.

Sponges have three layers: an external protective epithelial layer; a central protein-rich matrix called mesohyl with amoeboid cells; and an inner layer of choanocytes that circulate water and capture food particles.

Sponges have a loose body organization.

Sponges lack tissues and organs and a definite symmetry, but they do have a complex multicellularity. Larval sponges are free swimming and the adults are anchored onto submerged objects

Morphology-based phylogeny focused on the state of the coelom.

The 2 groups of bilaterally symmetrical animals (the Bilateria)- protostomes and deuterostomes- differ in embryology. Possession of a coelom is an ancestral character of the clade comprising the protostome and deuterostomes. All deuterostomes have a coelom, but some protostomes have evolved a pseudocoelom and other have lost a coelom entirely.

The Bilateria

The Bilateria are characterized by bilateral symmetry, which allows for functional specialization such as having nerve receptors at the anterior end of the body.

Eumetazoa: Animals with True Tissues

The Eumetazoa contains animals with distinct tissues. The embryonic cell layers differentiate into the tissue of the adult body, giving rise to the body plan characteristic of each group of animals.

Cnidarians are grouped into 5 classes.

The five classes of Cnidaria are the Anthozoa (sea anemones, corals, seafans); Cubozoa (box jellies); Hydrozoa (hydroids, Hydra), scyphozoa (jellyfish); and Staurozoa (star jellies). The staurozoa class is not accepted by all scientists.

Deuterostomes include chordates and echinoderms

The major groups of deuterostomes are the echinoderms, which include animals such as sea stars and sea urchins, and the chordates, which include vertebrates. Deuterostome development indicates that echinoderms and chordates evolved from a common ancestor, distinguishing them clearly from other animals.

Choanocytes help circulate water through the sponge.

The mesohyl may contain spicules and/or fibers of a tough protein called spongin that strengthen the body of the sponge.

Which of the following cell types of a sponge possesses a flagellum? a. Choanocyte b. Amoebocyte c. Epithelial d. Spicules

a. Choanocyte

With regard to classification of animals, the study of which of the following is changing our understanding of how animals are organized? a. Molecular systematics b. Origin of tissues c. Patterns of segmentation d. Evolution of morphological characteristics

a. Molecular systematics

A fluid-filled cavity that develops completely within mesodermal tissue: a. coelomate b. pseudocoelomate c. acoelomate d. All of the choices are correct

a. coelomate

The coelom a. is a synapomorphy for the clade compromising protostomes and deuterostomes b. evolved multiple times convergently c. is always an indicator of close evolutionary relationship d. forms within the endoderm

a. is a synapomorphy for the clade compromising protostomes and deuterostomes

In animal sexual reproduction the gametes are formed by the process of: a. meiosis b. mitosis c. fusion d. binary fission

a. meiosis

Which of the following characteristics would not apply to a species in the Ecdyozoa? a. Bilateral b. Indeterminate cleavage c. Molt at least once in their life cycle d. Metazoan

b. Indeterminate cleavage

Which of the following statements is NOT true regarding segmentation? a. Segmentation allows the evolution of redundant systems b. Segmentation is a requirement for a closed circulatory system c. Segmentation enhances locomotion d. Segmentation represents an example of convergent evolution.

b. Segmentation is a requirement for a closed circulatory system.

Animals are unique in the fact that they possess ____ for movement and ____ for conducting signals between cells. a. brains; muscles b. muscle tissue; nervous tissue c. limbs; spinal cords d. flagella; nerves

b. muscle tissue; nervous tissue

The larval stage of a cnidarian is known as a : a. medusa b. planula c. polyp d. cnidocyte

b. planula

Which of the following characteristics is unique to all animals? a. Sexual reproduction b. Multicellularity c. Lack of cell walls d. Hetrotrophy

c. Lack of cell walls

Which of the following cell layers is not necessary to be considered a eumetazoan? a. Ectoderm b. Endoderm c. Mesoderm d. All of the above are found in all eumetazoans

d. All of the above are found in all eumetazoans

A coelomate organism may have which of the following characteristics? a. Circulatory system b. Internal skeleton c. Larger size than a pseudocoelomate d. All of the choices are correct.

d. All of the choices are correct

Which of the following characteristics is used to distinguish between a parazoan and a eumetazoan? a. Presence of a true coelom b. Segmentation c. Cephalization d. Tissues

d. Tissues

Acoel flatworms: a. are closely related to other flatworms b. have evolved an acoelom c. are a member of the Parazoa d. are the sister taxon to the clade compromised of protostomes and deuterostomes

d. are the sister taxon to the clade compromised of protostomes and deuterostomes

The evolution of bilateral symmetry was a necessary precursor for the evolution of: a. tissues b. segmentation c. a body cavity d. cephalization

d. cephalization