The Radiata-Bilateria split Second branching in the evolutionary tree
Two very important characteristics are used to distinguish between the second bifurcation of metazoans Body symmetry Germinal layers
1. Body symmetry Body symmetry refers to the arrangement of body structures relative to some axis of the body. Animals that can be bisected or split along at least one plane are called symmetrical. Two types of symmetries common in metazoans: Radial symmetry Bilateral symmetry Asymmetrical: With no plane of symmetry e.g. many sponges have an irregular growth
1. Radial symmetry Radial symmetry body of an animal has a general form of a cylinder, with one main axis around which the various body parts are arranged. characteristic feature of lower animals Radial symmetry takes many forms: i) Biradial symmetry occurs where portions of the body are specialised and only two planes of symmetry sectioning can divide the animal into two halves. e.g. sea anemones
ii) Multiradiality. a) quadri-radial symmetry (four planesline symmetry) e.g. jellyfishes b) penta-radial symmetry (5 planes of symmetry) e.g. sea stars Adaptive significant feature of radial symmetry is that such animals can confront their environment in numerous directions.
Features of radial animals: i. sessile species (e.g. sea anemones) and pelagic species (e.g. jellyfish, ctenephores) ii.all lateral surfaces have equal likelihood of interacting with the environment iii. the top and bottom of the organism have very different specialised functions. -The lower surfaces provide a stable, concrete point of attachment to some solid surface, - The upper surfaces modified for gathering of resources (e.g. food). iv. Movement is very slow if any v. A radial animal has no front or back end.
2. Bilateral symmetry Most important type of body symmetry. The body parts are oriented about an axis that passes from the front (anterior) to the back (posterior) end. There is only one plane of symmetry, which passes along the axis of the body to separate right and left sides which are mirror images. This plane is called the midsagittal plane.
Features of bilateral animals Have controlled mobility. the anterior end of the body confronts the environment first. anterior end encounters food and danger and other stimuli, first anterior end associated with concentration of feeding and sensory structures called CEPHALIZATION CEPHALISATION is the development of sense organs and central nervous system (CNS) tissues=formation of head.
Development of the head is an adaptation for unidirectional movement The body of a bilateral symmetrical animal is differentiated into two ends and two sides Anterior - end of the animal where the head is located Posterior - usually end of the animal where the anus is located Dorsal - side of animal without mouth, where back is located Ventral - side of the mouth, where belly is located.
Body symmetry alone is not a fool proof criterion that we can use to distinguish this branching. The reason being that some higher organisms attained radial symmetry secondarily e.g. some members of the Phylum Echinodermata But remember echinoderms are higher up in the evolutionary ladder, in fact they are closely related to us They start off being bilateral at larval stage and then attain radial symmetry as an adaptation
Germinal layers To distinguish between Radiata and Bilateria use of germinal layers A germ layer is a collection of cells, formed during early embryonic development (embryogenesis) These are the cellular layers from which the organs and tissues of the body develop through further differentiation organogenesis
All animals more complex than sponges (eumetazoans) produce two or three primary tissue layers (sometimes called primary germ layers). radial animals, like cnidarians, have two called ectoderm and endoderm (diploblastic) bilateral animals, above cnidarians have three called ectoderm, mesoderm and endoderm (triploblastic)
Embryonic Tissue Layers: Diploblatic vs. Triploblastic
Development Fertilization leads to the formation of a zygote. During the next stage, cleavage, mitotic cell divisions transform the zygote into a tiny ball of cells called a blastula. This early embryonic form undergoes a massive reorganization called gastrulation forming a gastrula with either two or three layers (the germ layers). In all vertebrates, these are the forerunners of all adult tissues and organs. The appearance of the archenteron marks the onset of gastrulation. What forms from gastrulation? Ectoderm; endoderm and the mesoderm (only in triploblastic animals)
The Ectoderm The ectoderm is the start of a tissue that covers the body surfaces. It emerges first and forms from the outermost of the germ layers. What forms from ectoderm germ layer? Nervous system Outer part of integument In vertebrates, the ectoderm has three parts: external ectoderm, skin (along with glands, hair, nails); epithelium of the mouth and nasal cavity lens and cornea of the eye the neural crest, melanocytes; peripheral nervous system; facial cartilage; dentin (in teeth) neural tube. (Brain, spinal cord and motor neurons; retina)
The Endoderm Cells migrating inward along the archenteron form the inner layer of the gastrula, which develops into the endoderm. What forms from the endoderm? Gastrointestinal tract Respiratory tract Endocrine glands (including liver and pancreas)
The Mesoderm Mesoderm forms in the embryos of animals more complex than cnidarians. Some of the cells migrating inward to form the endoderm form an additional layer between the other two. This key innovation evolved hundreds of millions of years ago and led to the evolution of nearly all large, complex animals. The formation of a mesoderm led to the formation of a coelom. Organs formed inside a coelom can freely move, grow, and develop independently of the body wall while fluid cushions and protects them from shocks.
What forms from the mesodermal layer? Bones most of the circulatory system connective tissues of the gut and integuments Excretory tract Mesenchyme Muscles Peritoneum (lining of the coelom) Reproductive system Urinary system (Please note: Not all triploblasts produce all of the items listed)
Summary of different tissues and organs derived from germ layers