Mulitcellular Organisms

The Three Germ Layers and Organs

		The three germ layers (endoderm, mesoderm and ectoderm) encompass the precursors of all structures and organs of the entire body, and are generated by a process called gastrulation (occurs at the early stage of the embryonic development). Body cavities of animals become increasingly more complex as one ascends the evolutionary tree. Ctenophora and Cnidaria are diploblastic (a) and have two thin but well-differentiated tissue layers separated by mesoglea, a gelatinous material that protects the body and lines the
Figure 10-10 Body Plans [view large image]	Figure 10-11a Alternate Perspective [view large image]	gut. Flatworms have three primary tissue layers and are triploblastic but are without an inner cavity called a coelom (b).

Ectoderm is the outer tissue layer, mesenchyme and mesoderm are the middle tissue layers, and endoderm is the inner layer. Flatworms lack a circulatory system, so oxygen must be transported to their inner tissue layer by diffusion, and thus their bodies must remain flat. Animals with the next most complicated body plan have three tissue layers as well as hemocoelic, or blood, spaces between the tissue layers (c). The most complicated body plans have both hemocoelic spaces and coelomic spaces (body cavity) lined with mesodermal tissue (d). The coelomic cavity separates the gut cavity from the body wall, and thus allows the development of complex gut structures. The mesentery is a membrane that supports an organ or body part. In general, animals with only blood spaces are more frequent found in lower branches of the tree than are animals with coelomic spaces. Figure 10-10 shows these four types of body plan. Figure 10-11a shows the three types of triploblastic (three layers) with a different perspective.

The ectodermis is responsible for the production of much of the sense organs, nervous system, and surface skin (among other things), while the endoderm contributes to the growth of structures such as the stomach and intestines, the lungs, endocrine glands, and the digestive glands. The middle layer is used to build muscular, skeleton, excretory, and reproductive systems. The mesoderm thus allows animals to become more organized in their structure (Figure 10-11b). Cnidarians, for example, are organized mainly at the level of tissues, while those with three layers possess organs and elaborate organ systems. The body plan for the advance organisms is essentially tube-within-a-tube with the gut cavity inside the body cavity (the coelom). In our body the coelom is divided by the diaphragm and membranes into the

Figure 10-11b Cellular Differentiation

peritoneal cavity (contains intestines and reproductive organs), pleural cavities (contain lungs), and pericardial cavity (contains heart).

Fluid in cavity helps distribute food, wastes, hormones, etc. from one end of animal to the other.
Better distribution allows animal to grow larger.
A place to put things, like new organs.
Hydrostatic skeleton- pressure makes cavity rigid.

		All the vital body organs except for the brain are enclosed within the trunk or torso. The trunk contains two large cavities separated by a muscular sheet called the diaphragm. The upper cavity, known as the thorax or chest cavity, contains the heart and lungs. The lower cavity, called the abdominal cavity, contains the stomach, intestines, liver, and pancreas, which all play a role in digesting food. Also within the trunk are the kidneys and bladder, which are part of the urinary system, and the reproductive organs, which hold the seeds of new human life. Figure 10-12a depicts the front view of the body cavity; while Figure 10-12b shows some inner organs. The three kinds of germ layers have differentiated into more than 350 different kinds of tissues and organs in adults. Table 10-01 is a list of the human organ systems, together with its function, and major
Figure 10-12a Anatomy, Front [view large image]	Figure 10-12b Anatomy, Inside [view large image]	components. All body systems work together; none are independent.

Organ System	Functions	Components
Circulatory	Transports nutrients, gases (O₂, CO₂), hormones and wastes through the body	Heart, blood vessels and blood
Digestive	Breakdowns and absorbs nutrients for growth and maintenance	Mouth, esophagus, stomach, liver, pancreas, gallbladder, small and large intestines
Endocrine (Hormonal)	Relays chemical messages through the body for controlling physiological processes	Hypothalamus, pineal, pituitary, thyroid, thymus, pancreas and adrenal glands
Excretory	Filters out cellular wastes, toxins and excess water or nutrients from the circulatory system	Kidneys, ureters, bladder, urethra, lungs, sweat pores, and intestine
Immune	Destroys or removes invading microbes, viruses; the lymphatic system also removes fat, and excess fluids from the blood. Skin is the outermost defense against pathogens	Spleen, thymus, bone marrow, lymph, lymph nodes and vessels, white blood cells, T- and B- cells, skin
Musculo-skeletal	Supports and moves organism; also protects delicate internal organs and provides attachment sites for the organs.	Skeletal and smooth muscles; bones, cartilage, tendons, and ligaments
Nervous & sensory	Relays electrical signals, directs movement, controls physiological processes, and responses to environment	Brain, nervous system, and the five senses
Reproductive	manufactures cells for reproduction	Female: ovaries, oviducts, uterus, vagina, and mammary glands; male: testes, as deferens, seminal vesicles, penis, and prostate gland
Respiratory	Provides gas exchange between the blood and the environment	Nose, trachea, and lungs