Animal Architecture Levels of organization in organismal complexity. There are 5 major grades of organization each being more complex than the previous.

Levels of organization 1. Protoplasmic level: occurs in unicellular organisms. Organelles within the cell carry out specialized functions. Protozoans are examples. 2. Cellular level: Cells are aggregated and cells engage in a division of labor, being specialized for particular tasks. Colonial protozoan groups with distinct somatic and reproductive cells and sponges are examples. (Animals that are multicellular are referred to as Metazoans).

Levels of organization 3. Cell-tissue level: Similar cells aggregate into patterns or layers forming tissues. Nerve net in Cnidarians (e.g. jellyfish) is example of a tissue.

Levels of organization 4. Tissue-organ level: Organs are made up of more than one kind of tissue and have a specialized function. Flatworms (Platyhelminthes) generally represent this level having organs such as eyespots and reproductive organs, but their reproductive organs are organized into level 5 an organ system.

Flatworm (Turbellaria)

Levels of organization 5. Organ-system level: Organs work together to perform functions. Most complex level of organization. Examples of organ systems include circulatory, reproductive, digestive, respiratory. Most animal phyla exhibit this level of organization.

Animal symmetry There are three types of symmetry. Spherical Radial Bilateral

Animal symmetry Spherical symmetry occurs mainly among protozoans. Radial symmetry occurs among the Cnidarians (jellyfish) and Echinoderms (starfish, sea urchins). Bilateral symmetry commonest form of symmetry. Strongly associated with cephalization or development of a head with associated sensory and feeding apparatus.

A variety of descriptive terms are used to describe orientation in bilateral animals.

Development of body plans An animal’s body results from division of cells during embryonic development. Differences in developmental patterns have been used to classify more complex animals so an understanding of basic embryology is necessary to follow this.

Process of development Once an egg is fertilized it becomes a zygote. This cell divides into a large number of cells called blastomeres. Cleavage of cells proceeds until a fluid-filled hollow ball of cells is formed. This is a blastula. In multicellular animals other than sponges the blastula invaginates to begin forming the future gut. At this stage the embryo is a gastrula.

Process of development The invaginating layer of cells, which will give rise to the gut, form a germ layer called the endoderm. The endoderm surrounds and defines a body cavity called the gastrocoel. The cells not involved in forming the invagination constitute another germ layer the ectoderm. The ectoderm surrounds a cavity called the blastocoel.

gastrocoel

Process of development When the invaginating gastrocoel forms a complete tube by forming a second opening to the outside it is then called the gut. In the cnidarians (jellyfish, sea anemones) no second opening develops.

Process of development In most animals (but not cnidarians, which are two-layered or diploblastic) a third germ layer of cells called the mesoderm develops. The mesoderm gives rise to many internal organs. Organisms with mesoderm are called triploblastic having three germ layers.

Germ layers Endoderm: innermost germ layer of an embryo. Forms the gut, liver, pancreas. Ectoderm: Outer layer of cells in early embryo. Surrounds the blastocoel. Forms outer epithelium of body and nervous system. Mesoderm: Third germ layer formed in gastrula between ectoderm and endoderm. Gives rise to connective tissue, muscle, urogenital and vascular systems and peritoneum.

Process of development The way in which the mesoderm forms, and whether or not a cavity (called a coelom) develops within it, are important characters in deciphering the relatedness of animal groups.

Coeloms The coelom is a cavity entirely surrounded by mesoderm. A coelom provides a tube-within-a-tube arrangement which has many advantages: Allows flexibility in arranging visceral organs permits greater size and complexity by exposing more cells to surface exchange fluid-filled ceolom can act as a hydrostatic skeleton

Coeloms Triploblastic organisms (organisms with three germ layers including mesoderm fall into one of three different coelomic states: Acoelomate: mesoderm fills the blastoceol, no cavity occurs in the mesoderm. Flatworms and nemerteans. Pseudocoelomate: mesoderm lines only outer edge of blastocoel. No peritoneal lining develops. Nematodes and rotifers. Eucoelomate: Have a true coelom derived from mesoderm and lined with peritoneum. Arthropods, annelids, mollusks, echinoderms, vertebrates.

Both eucolomate

Protostomes and Deuterostomes Within the eucolomates there are two major evolutionary lineages that split early in the history of animals and follow quite different developmental pathways. These are the protostomes “mouth first” and deuterostomes “mouth second”.

Important differences in development between protostomes and deuterostomes The differences in development that distinguish the protostomes and deuterostomes include: Whether cleavage of cells in the early zygote is spiral or radial. Whether or not, if the early blastomere is separated, each cell can develop into a normal larva or not. Whether the blastopore ultimately forms the mouth or anus of the organism. Whether or not the organism possesses a coelom and how that coelom is formed.

Figure 08.10

Protostomes and Deuterostomes Protostomes include the annelids, mollusks, and arthropods. Deuterostomes include the echinoderms and vertebrates.