Examples of acoelomates include animals in the phylum Platyhelminthes, also known as flatworms. Triploblasts that do not develop a coelom are called acoelomates, and their mesoderm region is completely filled with tissue, although they do still have a gut cavity.

The coelom also provides space for the diffusion of gases and nutrients, as well as body flexibility, promoting improved animal motility. In addition, organs housed within the coelom can grow and move freely, which promotes optimal organ development and placement. For example, the coelom provides cushioning and shock absorption for the major organ systems that it encloses. The evolution of the coelom is associated with many functional advantages. In the thoracic cavity further subdivision produces the pleural cavity, which provides space for the lungs to expand during breathing, and the pericardial cavity, which provides room for movements of the heart. In mammals, the body cavity is divided into the thoracic cavity, which houses the heart and lungs, and the abdominal cavity, which houses the digestive organs. It houses many organs such as the digestive, urinary, and reproductive systems, the heart and lungs, and also contains the major arteries and veins of the circulatory system. This epithelial cell-lined coelomic cavity, usually filled with fluid, lies between the visceral organs and the body wall. Presence or Absence of a Coelomįurther subdivision of animals with three germ layers (triploblasts) results in the separation of animals that may develop an internal body cavity derived from mesoderm, called a coelom, and those that do not. Diploblastic animals may have cell types that serve multiple functions, such as epitheliomuscular cells, which serve as a covering as well as contractile cells.

This germ layer gives rise to all specialized muscle tissues (including the cardiac tissues and muscles of the intestines), connective tissues such as the skeleton and blood cells, and most other visceral organs such as the kidneys and the spleen. The mesoderm is the third germ layer it forms between the endoderm and ectoderm in triploblasts. The ectoderm develops into the outer epithelial covering of the body surface, the central nervous system, and a few other structures. Generally speaking, the endoderm gives rise to the lining of the digestive tract (including the stomach, intestines, liver, and pancreas), as well as to the lining of the trachea, bronchi, and lungs of the respiratory tract, along with a few other structures. Animals with three tissue layers are called triploblasts.Įach of the three germ layers is programmed to give rise to specific body tissues and organs, although there are variations on these themes. More complex animals (usually those with bilateral symmetry) develop three tissue layers: an inner layer (endoderm), an outer layer (ectoderm), and a middle layer (mesoderm). The four clades considered to be diploblastic have different levels of complexity and different developmental pathways, although there is little information about development in Placozoa. These animals are called diploblasts, and have a nonliving middle layer between the endoderm and ectoderm (although individual cells may be distributed through this middle layer, there is no coherent third layer of tissue). The animals that display radial, biradial, or rotational symmetry develop two germ layers, an inner layer ( endoderm or mesendoderm) and an outer layer ( ectoderm). Animals develop either two or three embryonic germ layers (Figure 1). Recall that these germ layers are formed during gastrulation, and that each germ layer typically gives rise to specific types of embryonic tissues and organs. Most animal species undergo a separation of tissues into germ layers during embryonic development.

Compare and contrast the embryonic development of protostomes and deuterostomes.

Blastopore formation