There are three characteristics of animals that are not found in other multicellular organisms.
The colonial flagellate hypothesis relates to the origin of animals.
Animals are in the domain Eukarya.
In this section, we look at how the characteristics of animals evolved.
Animals, fungi, and plants are multicellular organisms, but unlike plants, they need an external source of nutrition.
Animals and saprotrophs are different because animals eat food and digest it internally, while saprotrophs absorb and excrete food.
The cell walls in plants and fungi are made of different organic molecules, but animal cells lack a cell wall.
Animals are mobile, have nerves and muscles, and reproduce sexually.
It is difficult to assign specific characteristics to all animals.
There are a variety of life cycles for animals.
Some people reproduce sexually and some people reproduce asexually.
Many animals have a diploid life cycle, but other life cycles, such as haploid or haplodiploid, are also represented.
Animals go through a series of stages in their development to produce an organisms with specialized tissues.
Animals are able to search for food and prey on other organisms.
Animals have been allowed to live in all habitats and become vastly diverse because of coordinated movements.
Animals form a single line on the tree of life from a single common ancestor.
The animals have two main branches, the vertebrates and the invertebrates.
All animal cells lack a wall.
Heterotrophs are animals that get their nutrition from outside sources.
The nervous and muscular systems of animals make them motile.
Most animals reproduce sexually, beginning life as a 2n zygote, which undergoes development to produce a multicellular organism that has specialized tissues.
There is evidence that plants most likely share a protist ancestor with the charophytes.
The evidence shows that they also evolved from a protist.
The colonial flagellate hypothesis states that animals are descended from a colony of flagellated cells.
The choanoflagellates are the closest living relatives of animals and are most likely to resemble the last single-celled ancestor of animals.
A choanoflagellate is a single cell with a flagellum surrounded by 40 microvilli.
The water moves through the microvilli.
A bunch of grapes can be found at the end of a stalks.
The figure shows the process of transition from colonial flagellates to multicellular animals.
A large number of cells could have formed a hollow sphere.
Individual cells within the colony would become specialized for reproduction.
Two layers of tissue could have arisen from a hollow sphere.
During the development of animals today, tissue layers arise.
The colonial flagellate hypothesis has implications regarding animal symmetry, which will be discussed soon.
The numbered statements explain the hypothesis that motile flagellates may evolve into a multicellular organisms with specialized cells and more than a single cell layer.
The animal body plans we see today are from 500 million years ago.
There are many possible outcomes for the number, position, size, and pattern of the animal's body parts.
The variety of animal forms in the past and present could have been a result of different combinations.
The fossil record of the early evolution of animals is very sparse.
The best hypothesis of the evolution of animals is based on a combination of the characters of living and fossil organisms.
Two organisms are more similar than not.
The addition of data has resulted in updated trees that are different from the ones before.
The colonial flagellated protist who lived about 600 mya is believed to have descended from the animal phyla.
The hypothesis of this tree is to determine which phyla are most closely related to one another.
The placement of Ctenophora and relationships among Spiralia are most uncertain.
Refer to Figure 28.6 as we discuss the differences between animals in the animal family tree.
There are three types of symmetry in the animal kingdom.
The bodies of the cnidarians and ctenophores are similar to a wheel in that they are circular and have two identical halves.
There are many forms of animals that are symmetrical.
The organisms are able to extend out in all directions from one center.
Animals with radial symmetry can be floating.
Only a single longitudinal cut down the centerline of the animal produces two equal halves as the rest of the animals exhibit bilateral symmetry.
Bilaterally symmetrical animals have defined ends, and forward movement is guided by the anterior end.
The colonial flagellate hypothesis is attractive because it implies that radial symmetry preceded bilateral symmetry in animal history.
This development was important to an animal's ability to move away from danger.
sponges are the simplest animals.
sponges are multicellular They do not have specialized tissues.
embryological development leads to the development of true tissues in more complex animals.
The first tissue layers that appear are called germ layers, and they give rise to the organs and organ systems of complex animals.
There are no specialized organs in diploblastic animals.
Placozoa is a poorly known phylum with only a single living member discovered.
The mouth or the anus develops first in the embryo.
The anus develops prior to the mouth in deuterostomes.
All animals, including humans, are deuterostomes.
Protostomes are more similar to deuterostomes.
Each cell has limited potential and cannot develop into a complete embryo, and the blastopore is associated with the mouth.
New cells sit on top of old cells to form a complete embryo, and the blastopore is associated with the anus, if present.
The daughter cells sit in grooves formed by the previous cleavages in most protostomes.
The fate of these cells is determined by the way in which they contribute to development.
The daughter cells sit on top of the previous cells in deuterostomes.
The fate of these cells is uncertain, because if they are separated from one another, each cell can become a complete organisms.
As development progresses, a hollow sphere of cells forms and an opening called the blastopore is created.
The mouth appears near the blastopore.
The anus appears at or near the blastopore in deuterostomes.
All deuterostomes have a body cavity lined by aderm.
The coelom in these groups is due to the fact that the mesoderm cells line the cavity completely.
In the two groups, the coelom develops differently.
The coelom can be produced by a splitting of cells located near the blastopore.
The coelom can be found in the wall of the primitive gut.
The pouch enlarges until they meet.
Chapter 42 tells you more about animal development.
The animal body plan can be divided into three categories based on symmetry, from a lack of symmetry in the sponges, to radial symmetry in the cnidarians, to bilateral symmetry in more recently evolved groups.
The body plan of an animal is the result of a carefully orchestrated pattern of genes being expressed at the right time and in the correct region of the developing embryo.
The anterior and rear ends of the embryo are determined in the first stage of development.
The next step in development is to divide the embryo into segments, each of which will become a different part of the body.
There is a plan on Page 517 during development.
When, where, and how long a particular developmental gene is active are all controlled by homeotic proteins.
Some animals have more than one cluster of genes with duplicate copies.
The fruit fly embryo is sectioned and stained during different stages of development.
The chicken and mouse have 7 and 12 thoracic vertebrae, respectively.
There are three characteristics that all animals have in common.