General Animal Traits, Development, and Origins

Question 1

What is the definition of an animal?

Answer 1

Animals are multicellular, heterotrophic eukaryotes with tissues that originate from embryonic layers.
- However, there are exceptions to nearly every criterion used to distinguish animals from other life forms.

Question 2

Characteristics of animals

Answer 2

Cell Structures
- Multicellular eukaryotes
- No cell walls
- Multiple cell types and tissue

Nutritional Mode
- Phagotropic chemoheterotrophs

Reproduction
- Mostly sexual reproduction

Development
- Involves blastulation and gastrulation
- Involves Hox genes
- Direct or indirect development

Question 3

Tissues

Answer 3

Tissues consist of groups of specialized cells that share a common structure and function.

Question 4

Cell structure and specialization

Animals

Answer 4

Animals are multicellular eukaryotes, except for gametes.

Animals lack cell walls found in other multicellular eukaryotes (plants, algae, and fungi).
- Tissues

Animal somatic (non-reproductive) cells differentiate into specialized types, such as those involved in digestion, secretion, protection, and transport.

Question 5

What types of tissues are found in animals

And only animals

Answer 5

The number of cell types varies widely among animals, from about 4-5 in simple organisms like sponges to >200 in more complex organisms like humans

Animals possess specialized cell types not found in other multicellular organisms:
- Neurons(nervecells),which generate and conduct nerve impulses, are components of nervous tissues.
- Contractile muscle cells, which form different types of muscle tissues, are responsible for body movement.

Nervous tissue and muscle tissue are defining characteristics of animals.

Question 6

Nutritional mode

Animals

Answer 6

Animals are chemoheterotrophs, relying on preformed organic molecules for both carbon and energy (as are fungi).
- Animals cannot construct all their organic molecules; they obtain these organic molecules by consuming other organisms.

Animals are phagotrophic heterotrophs.
- Animals ingest and digest food particles internally.
- cf. external digestion in fungi (absorptive heterotrophs).

Question 7

Animal Sexual Reproduction

Answer 7

Most animals reproduce sexually, with the diploid (2n) stage dominating the life cycle.
- cf. haploid-dominated life cycle in fungi.
- While animals may have multiple life stages, some of which reproduce asexually, all stages are diploid.

Sexual reproduction involves meiosis, producing haploid (1n) gametes that fuse during fertilization to form a diploid (2n) zygote.
- Animals produce gametes of different sizes: female gametes (eggs) are large, non-motile cells; male gametes (sperm) are smaller, motile cells.

Question 8

Can animals reproduce asexually?

Answer 8

Many animals reproduce asexually, generating genetically identical offspring from a single parent without fertilization (fusion of gametes).
- Asexual reproduction is found in nearly half of all animal phyla.

Question 9

Mechanisms of asexual reproduction

Animals

Answer 9

Mechanisms of asexual reproduction:

Fission or fragmentation occurs commonly in invertebrate animals.
- The animal splits into two or more parts that regenerate into complete organisms.

Budding, the formation of new individuals from outgrowths of existing ones, is found only among invertebrates.

Parthenogenesis is the development of an embryo from an unfertilized egg cell.
- Parthenogenesis is found among invertebrates and vertebrates

Question 10

Benifits of asexual reproduction

Animals

Answer 10

• Supports rapid population growth when conditions are favourable
• Provides an alternative to sexual reproduction when reproductive opportunities are limited
• Some species can alternate between sexual and asexual strategies

Question 11

Stages of embryonic development

Answer 11

1. Cleavage
2. Blastulation
3. Gastrulation
4. Regulated by conserved Hox genes

Question 12

Cleavage

Animal development

Answer 12

Following fertilization, the diploid zygote undergoes a series of rapid mitotic cell divisions called cleavage, transforming the zygote into a solid ‘ball’ of cells (morula

Question 13

Blastulation

Animal Development

Answer 13

In most animals, continued cleavage transforms the morula into a multicellular, hollow blastula (blastulation).
- The blastula is typically a hollow ball of cells surrounding a central fluid-filled cavity called the blastocoel (coel = opening).
- The blastula stage of embryonic development is found only in animals

Question 14

Gastrulation

Answer 14

Most animals also undergo gastrulation (gaster = stomach), forming a gastrula with different layers of embryonic tissues.
- Cells from one end of the blastula fold inwards, filling the blastocoel, producing two layers of embryonic tissues: the ectoderm (outer layer) and the endoderm (inner layer) (derm = skin).
- Gastrulation is unique to animals; it’s not found in fungi or other multicellular eukaryotes.
- The cavity formed by gastrulation, called the archenteron, opens to the outside via the blastopore.

Question 15

Role of Hox genes

Answer 15

Hox genes are crucial in animal evolution because they play a fundamental role in determining the body plan and segment identity during embryonic development.
- Hox proteins coordinate the development of various structures along the anterior-posterior axis, e.g. legs, antennae, and wings in fruit flies, or the different types of vertebrae in humans.
- The Hox family of genes is highly conserved.
- The arrangement of Hox genes along chromosomes remains similar across animal phyla.
- However, the number of Hox copies varies among animal phyla, e.g. least in jellyfish, most in vertebrates.
- Additionally, many other developmental genes are conserved among animals, contributing to the regulation of embryonic development.

Question 16

Direct vs Indirect development

Answer 16

Direct development is where the animal after birth or emergence from an egg is a smaller version of its adult form.
- A juvenile resembles an adult but is not yet sexually mature, e.g. humans.
- No larval stages or metamorphosis.

Indirect development has intervening stages (larvae) with morphological and behavioural differences from the sexually mature adult stage, e.g. caterpillar → butterfly.
- Most animals have at least one larval stage.
- A larva is sexually immature and morphologically distinct from the adult; it eventually undergoes metamorphosis to become a juvenile.

Question 17

Animal Motility in Development

Answer 17

Animals are motile: they are capable of moving their entire multicellular body using metabolic energy in at least one stage in their life cycle.
- Many marine animals with sessile (immobile) adult forms have motile larval stages in their development.

Question 18

Radial Symmetry

Answer 18

Radial symmetry: Some animals exhibit radial symmetry, where their body is arranged around a single main axis that passes through the centre of the animal.
- Radially symmetrical animals can be divided into numerous planes of symmetry, e.g. sea anemones.
- Radially symmetrical animals are often sessile or planktonic (drifting or weakly swimming).

Question 19

Bilateral Symmetry

Answer 19

Bilateral symmetry: Most animals display bilateral symmetry, with a distinct left and right side and a single plane of symmetry along a head-tail axis, e.g. lobster.

Bilateral symmetry associated with:
* Cephalization, the development of a head region containing sensory organs.
* Specialized appendages for directional movement, grasping, or defence.
* Bilaterally symmetrical animals tend to be more active and possess a centralized nervous system

Question 20

What is bilateral symmetry usually associated with

Answer 20

• Cephalization, the development of a head region containing sensory organs.
• Specialized appendages for directional movement, grasping, or defence.
• Bilaterally symmetrical animals tend to be more active and possess a centralized nervous system

Question 21

What are the three germ layers

Answer 21

During development, distinct embryonic cell layers (germ layers) give rise to tissues and organs of animal embryos:
- Ectoderm is the germ layer covering the embryo’s surface and gives rise to the skin and nervous system.
- Endoderm is the innermost germ layer and lines the developing digestive tube, the archenteron.

Many animals also have a mesoderm, which develops into muscle tissues

Question 22

Diploblastic animals

Answer 22

Diploblastic animals have two embryonic cell layers: ectoderm and endoderm.
- Radially symmetrical animals are diploblastic; includes cnidarians and a few other groups.

Question 23

Triploblastic Animals

Answer 23

Triploblastic animals have an additional intervening mesoderm (meso = middle) layer that gives rise to muscles and other organs.
- Bilaterally symmetrical animals are triploblastic: three embryonic cell layers (ecto-, endo-, and mesoderm).
- Most animals are bilaterians (triploblastic), e.g. flatworms, arthropods, vertebrates, and others.

Question 24

Which animals develop a coelom?

Answer 24

Larger animals develop a coelom during embryonic development of the mesoderm.
- The coelom is lined by mesodermal tissue, forming between the outer body wall (ectoderm) and the digestive tract (endoderm).
- Coeloms contain coelomic fluid.

Coelom function varies between animals:
- Inner and outer layers of mesoderm that surround the coelom connect and form structures that suspend the internal organs.
- Allows internal organs to shift without deforming outside of the body, e.g. the digestive tract’s movement and heart beating.
- Cushions internal organs from external impacts.
- A fluid-filled coelom often functions as a hydrostatic skeleton in soft- bodied animals by tensing muscles against the incompressible coelomic fluid, e.g. earthworms.

Question 26

Which animals have a hemocoel?

Answer 26

Many triploblastic animals have a hemocoel **A hemocoel forms between the mesoderm and the endoderm.** - The hemocoel arises from the blastocoel, the embryonic cavity of the blastula/gastrula. - Animals with hemocoels retain the blastocoel cavity during mesoderm development to form the hemocoel. - Because the hemocoel has a simple embryonic origin, hemocoels have evolved independently in many animal groups. Hemocoels contain **hemolymph** (fluid) (hemo = blood). - Hemolymph is analogous to blood and is circulated throughout the body cavity in an open circulation system by the heart. - The hemocoel and hemolymph are involved with internal circulation, nutrient transport, and waste removal, but can also function as a hydrostatic skeleton in some animals. Since coeloms and hemocoels have different embryonic origins, both can be found in some animal groups.

Question 27

# True or False All triploblastic animals have a body cavity

Answer 27

False Some triploblastic animals are compact and **lack a body cavity**. - These are small, flat animals, e.g. flatworms (Platyhelminthes). - They do not require an internal transport and circulation system, instead relying on diffusion across the body surface.

Question 28

What are coelomates

Answer 28

Animals possessing coeloms are sometimes called coelomates. - But coeloms (and hemocoels) have been reduced or lost in several groups. - The presence or absence of coeloms and hemocoels is not a good indicator of phylogenetic relationships. - Animals with coeloms or hemocoels do not form clades

Question 29

Which ways do protostome and deuterostome development differ?

Answer 29

**Embryo cleavage** - Protostome: Spiral and determinate - Deuterostome: Radial and indeterminate **Coelom formation** - Protostome: Solid masses of mesoderm split and form coelom - Deuterostome: Folds of archenteron **Fate of the blastopore** - Protostome: Mouth develops from blastopore - Deuterostome: Anus develops from blastopore

Question 30

protostome development - cleavage

Answer 30

In protostome development, cleavage is **spiral and determinate**. - Each new row of cells is twisted slightly off- center (spiral). - Determinate cleavage means that each new cell is predetermined to form a specific part of the later embryo. - Removal of some cells results in an incomplete embryo, e.g. missing specific organs.

Question 31

deuterostome development - cleavage

Answer 31

In deuterostome development, cleavage is **radial and indeterminate**. - Each cell division stacks the new cells directly above the previous ones. - Indeterminate cleavage allows each cell in the early stages of cleavage to retain the capacity to develop into a complete embryo. - Indeterminate cleavage enables the formation of identical twins and embryonic stem cells.

Question 32

Embryonic development – Coelom formation

Answer 32

In **protostome development**, the coelom forms through the splitting of solid masses of mesoderm. In **deuterostome development**, the mesoderm folds from the wall of the archenteron to form the coelom.

Question 33

Embryonic development – Fate of the blastopore

Answer 33

The **blastopore** forms during gastrulation and connects the archenteron to the exterior of the gastrula. In **protostome development**, the blastopore becomes the mouth (proto = first, stoma = mouth). In **deuterostome development**, the blastopore becomes the **anus**, while a second invagination forms the mouth (deutero = second).

Question 34

Key features of the phylogeny of extant animals

Answer 34

1. All animals share a single common ancestor, an ancestral colonial flagellated protist. − **Kingdom Animalia** constitutes **clade Metazoa**, multicellular animals. 2. Sponges are basal animals in the phylogeny. 3. **Eumetazoa** (“true animals”) is a clade of animals with true tissues. 4. Most animal phyla belong to the **clade Bilateria**, animals with bilateral symmetry (**bilaterians**). 5. There are t**hree major clades of bilaterian animals**, all of which are **invertebrates**, animals that lack a backbone, except most of Chordata, which are classified as vertebrates due to the presence of a backbone.

Question 35

What are the 3 major clades of bilaterian animals

Answer 35

1. Deuterostomia (includes: hemichordates (acorn worms), echinoderms (starfish and relatives), and chordates) 2. Ecdysozoans (includes arthropods) 3. Lophotrochozoans (includes: jellyfish)

Question 36

Deuterostomia

Answer 36

Deuterostomia includes hemichordates (acorn worms), echinoderms (starfish and relatives), and chordates. - Deuterostomia exhibits deuterostome embryonic development and includes both vertebrates and invertebrates.

Question 37

Ecdysozoans

Answer 37

Ecdysozoans are invertebrates that undergo ecdysis, the process of shedding (moulting) their exoskeletons. - e.g. Arthropoda, which includes insects, arachnids, crustaceans, and others.

Question 38

Lophotrochozoans

Answer 38

Lophotrochozoans - Some have a feeding structure called a lophophore, a tentacle- covered feeding structure, while others exhibit a distinct larval stage called a trochophore larva. - e.g. molluscs and annelids - Some Lophotrochozoan phyla exhibit characteristics of both proto- and deuterostome development.

Question 39

When did the common ancestor for animals exist?

Answer 39

Molecular phylogenetic analyses indicate that the common ancestor of animals evolved between 650 and 800 million years ago (mya). - This ancestral protist likely **resembled modern choanoflagellates**, which are flagellated eukaryotes. - Choanoflagellates are opisthokont protists, the sister group to clade Metazoa (animals).

Question 40

Choanoflagellate protists

Answer 40

Extant choanoflagellates are small, unicellular, heterotrophic protists. - >125 species of unicellular or colonial forms (stalked or ball-like colonies) found in marine and freshwater environments. - Choanoflagellates are **collared flagellates** (choano = collar) with a **funnel-shaped microvilli collar** at the flagellum’s base. - Microvilli are finger-like projections of the cell membrane that capture bacteria. - Animals are hypothesized to have evolved from choanoflagellate-like ancestors

Question 41

What is the evidence that choanoflagellates are closely related to animals

Answer 41

1. **Cell morphology**. Choanoflagellate cells and the collar cells of sponges are almost indistinguishable 2. **Cell morphology unique to animal cells**. Cells resembling choanoflagellates are found in other animals, but never in non-choanoflagellate protists, plants, or fungi. 3. **DNA sequence homology** Molecular phylogenies confirm choanoflagellates as the closest extant relatives of animals

Question 42

# True or False All multicellular life has one common ancestor

Answer 42

Multicellularity evolved independently in multiple lineages, leading to the development of algae, plants, fungi, and animals. - The oldest multicellular eukaryote fossils (algae) are from ~1.2 bya. - The **evolutionary advantages of multicellularity** include cell specialization, increased size and complexity, longer lifespans, and enhanced defense against predation. - Molecular clock calculations suggest that **animals originated around 650–800 mya**. - Whole-body animal fossils date from the Ediacaran Period (Neoproterozoic Era; Fig 25.11).

Question 43

Neoproterozoic Era | Animals

Answer 43

The earliest complex, multicellular animal fossils are from the **Ediacaran biota** (635–540 mya). - The rise of marine planktonic algae (Archaeplastida) ~800–650 mya increased atmospheric and oceanic oxygen levels (**Neoproterozoic Oxygenation Event**). - The transition to algal-dominated ecosystems created food webs with more efficient nutrient and energy transfers, supporting the evolution of larger and increasingly complex organisms. - The Ediacaran biota were diverse soft-bodied, mostly non- motile (sessile) marine organisms. - Ediacaran biota was largely extinct by the Cambrian

Question 44

Paleozoic Era | Animals

Answer 44

The **Cambrian explosion**, from 535 to 525 mya, marked a significant diversification in animal evolution. - Animal fossils from the Cambrian explosion show a rapid **increase in the diversity of bilaterian animals** with bilateral symmetry and complete digestive tracts. - While many extant animal phyla appeared during the Cambrian explosion, some, like sponges, cnidarians, and molluscs, preceded it.

Question 45

Hypotheses for the Cambrian explosion

Answer 45

**Evolution of predation**. - The evolution of predator-prey relationships led to the evolution of more complex body structures and defence mechanisms. **Neoproterozoic Oxygenation Event**. - Increase in oceanic [O2] supported greater body size and more energetic lifestyles, e.g. active, muscular carnivores. **Evolution of the Hox genes**. - The emergence of Hox genes, which control the body plan of animals, facilitated the diversification of body forms during the Cambrian explosion.

Question 46

Animal evolution per era

Answer 46

**Paleozoic Era (541–252 mya)**: - Animal diversity increased during the Paleozoic but was punctuated by the Ordovician, Late Devonian, and Permian mass extinctions. - Invertebrates began colonizing land ~450 mya, followed by vertebrates around 365 mya. **Mesozoic Era (252–66 mya)**: - Dinosaurs were dominant terrestrial vertebrates during the Mesozoic Era. - The first mammals emerged. **Cenozoic Era (66 mya to present)**: - Mammals increased in size and diversity after the Cretaceous mass extinction, exploiting available ecological niches.

Question 47

Invertebrates

Answer 47

Invertebrates consist of 95% of known animal species Invertebrates occupy almost every habitat on Earth and include a diverse range of species, ranging from microscopic to huge. - e.g. colossal squid (Mollusca): largest specimen, 495 kg, 10 m. The term “invertebrates” refers to animals without a backbone. - Anatomical classification. - Invertebrates are paraphyletic, i.e. not a clade

Question 48

Shared derived traits of clade Metazoa

Answer 48

1. Multicellularity and cell differentiation 2. Cell adhesion (extracellular matrix) 3. Sperm and ova (egg) 4. Embryonic blastula*

Question 49

Shared derived traits of Clade Eumetazoa

Answer 49

1. True tissues, include muscle and nerve tissues 2. Gastrulation 3. Diploblastic 4. Radial symmetry

Question 50

Shared derived traits of Clade Bilateria

Answer 50

1. Bilateral symmetry 2. Triploblastic 3. Complex organs