The below mentioned article provides a detailed account of Metazoa.
Protozoa are acellular animals of small size, they have a permeable limiting membrane which prevents the animals from growing beyond a certain size, and it precludes the formation of structures which will give strength and rigidity necessary for large size.
But more important than strength is the factor that the activities of Protozoa involve an exchange of substances between their protoplasm and their surrounding liquid medium, these processes are governed by the ratio of their surface area to their volume; the smaller the animal, the relatively larger is its surface area, this ratio limits their size.
In Protozoa the small acellular body performs all vital functions, and no single function predominates over the others.
The attainment of multicellular structure is foreshadowed by colonial Protozoa, they have groups of many individuals either attached to each other, or connected to each other by protoplasmic strands, or embedded in a common matrix of non-cellular material. But colonies are different from multicellular animals because their cells are functionally independent of each other.
The larger multicellular animals in which limitations of size are removed are called Metazoa, their cells are potentially capable of performing all essential vital activities, but these cells are dependent on each other, and all of them are not similar because specialisation has taken place, this has opened up vast possibilities for an increased complexity of body form and structure.
Cell specialisation in turn has led to the development of tissues in which groups of similar cells are organised into sheets or layers. In lower metazoans the tissues are primitive and cells arranged in layers, but in higher forms the tissues become organised to form organs and organ systems.
Metazoa are multicellular animals which are distinguished not only by their larger size, but by a high degree of differentiation and specialisation of their parts, this is called morphological differentiation.
With this morphological differentiation of structure, there is a physiological division of labour among the permanently associated and mutually dependent parts of an animal; this implies that parts of the body are specialised to perform definite functions for the entire animals. Differentiation of structure is also seen in many Protozoa, but it attains a much higher degree of complexity in Metazoa.
In Metazoa, special regions of the body are set aside for dealing with different functions. The Metazoa produce gametes of two types, the male gametes are spermatozoa and female gametes are ova. A spermatozoon fertilises an ovum to form a zygote which undergoes a series of mitotic cell divisions to form a hollow ball of cells called blastula it has a cavity known as blastocoele.
Further increase of the cells of the blastula causes an invagination of the wall on one side, and by different processes the cells eventually come to lie in two layers, an outer layer of ectoderm and an inner layer of endoderm, the blastocoele is obliterated; the mouth of the invagination is a blastopore which leads into a new cavity, the archenteron; this two-layered bag is a gastrula.
The development of some Metazoa stops at the gastrula stage, these two-layered Metazoa are diploblastica, such as Cnidaria and Ctenophora. In all other metazoan phyla, a third layer of cells called mesoderm arises between the ectoderm and endoderm.
The phyla which possess three layers are triploblastica, and their mesoderm opens up further possibilities of increase in size and complexity. In triploblastic phyla, the ectoderm and endoderm retain most of the functions which they perform in diploblastic animals.
The ectoderm forms the outer protective epidermis, external sense organs, nephridia and the nervous system, but in Echinodermata, part of the nervous system is mesodermal in origin. The endoderm gives rise to the lining of the alimentary canal, and organs associated with digestion and respiration.
The mesoderm is not single entity but has parts which originate in two ways; the cells which migrate from the ectoderm or endoderm form a loose cellular tissue called mesenchyme which fills the spaces between the other layers, and it is comparable to the cells which are found in the mesogloea of Cnidaria.
The second type of cells which form the wall of the body cavity are known as mesothelium or real mesoderm which gives rise to connective tissue, muscles, skeleton, blood, circulatory system, excretory system and reproductive system. In lower triploblastic phyla (Platyhelminthes and Aschelminthes), there is no mesothelium, in Chaetognatha there is no mesenchyme, but the other phyla possess both kinds of mesoderm.
The triploblastic acoelomate animals may attain a degree of complexity not seen in diploblastic animals, though the gut of some triploblastic acoelomate animals has only a single opening, the mouth which serves both for ingestion and egestion.
The diploblastic animals are no doubt simpler, but the higher diploblastic animals approach a condition found in the lower triploblastic animals in having what amounts to third layer of cells, i.e., cells in the mesogloea; thus, the distinction between diploblastic and triploblastic animals is by no means very sharp.
In Metazoa, the cells are closely associated to form tissues which are specialised for performing some functions. Cnidaria have no organised tissues, but in triploblastic animals, the tissues unite to form organs, the organs are then associated together to form systems, each of which carries on some important general function, though there may be some overlapping.
The epidermis along with underlying mesodermal tissue called dermis forms the skin; in invertebrates it is either columnar or syncytial; in vertebrates it is stratified. In lower invertebrates the cells of the “skin” are ciliated, when cilia are absent then a protective cuticle is formed.
A special region of the body is set aside for dealing with food, it may be a simple sac or a complicated alimentary canal for enclosing the food and secreting digestive enzymes. Digestion is originally entirely intracellular in endoderm cells, this may be preceded by an extracellular digestion, but in higher phyla (Annelida, Arthropoda, some Mollusca and Chordata) it is entirely extracellular.
The enzymes secreted by the animal render the major part of the food soluble and capable of absorption and assimilation. Such a digestive system is essential for larger animals, for they require such large quantities of food for their vital activities that it cannot be taken into food vacuoles.
In large Metazoa parts of the body lie at some distance from the digestive system, so they cannot receive nourishment by mere diffusion, as is done in lower Metazoa, hence, they require a transporting system of tubes (as in jelly fish) or a blood vascular system (as in most higher animals) which can transport the digested food.
In early stages, stimuli are transmitted from sensory cells or receptors to muscles or other cells called the effectors which are set in action. But in larger Metazoa where the effectors may lie at some distance from the receptors, it became essential to have a system for concluding and coordinating, thus, a neurosensory system is developed.
This is done by formation of nerve cells or neurons which have several branching cytoplasmic processes called nerve fibres. In its simplest form, the neurosensory system would have a series of receptors on the body surface from which nerve fibres pass to the effectors.
But such a simple system does not exist, there is no direct connection between receptors and effectors, but conduction takes place through a chain of neurons; the neurons are not joined but there are minute terminal buttons at the end of an axon which lie in contact with the dendrites of the next neuron, these junctions are called synapses.
In lower Metazoa, the nerve fibres intermesh to form a network which is superficial in position and is called a nerve net. In higher Metazoa, the nerve fibres of a neuron are not equally formed on all sides, one or more of them are long, and the fibres are bound together to form nerves, and the cell-bodies of neurons become collected together to form a central nervous system.
The nerves which connect the central nervous system to receptors and effectors constitute a peripheral nervous system. Thus, impulses are conducted from receptors along definite paths and not in any direction, as in a nerve net.
Most of the smaller Metazoa are aquatic, their relatively large surface provides an adequate area for interchange of gases necessary for respiration, and allows nitrogenous waste substances to diffuse out quickly. The larger Metazoa have a relatively smaller surface area, and they may have an external covering, hence, they form respiratory organs.
These organs of respiration may be covered or lined by ectoderm (gills of crustaceans and annelids, external gills of tadpole and lungs of snails); or they may be covered by endoderm (gills of fish and lungs of vertebrates). The skin in many small and large animals is respiratory.
Aquatic respiration is affected by pressure changes of oxygen in water, aerial respiration is affected by pressure changes of carbon dioxide, in foul water the amount of free carbon dioxide is so large as to be an important factor.
The organs of excretion are very varied, they are needed for removal of carbon dioxide, water and solid nitrogenous waste substances.
The excretion may occur through the body surface through the ectoderm and perhaps also endoderm (Cnidaria), or in triploblastic animals by a large excreting surface inside a complicated system of fine, branching tubules which form ectodermal nephridia or mesodermal uriniferous kidney tubules, both of which open directly or indirectly to the exterior.
The triploblastic animals have a rigid skeleton for support and attachment of body muscles. In Arthropoda, there is an exoskeleton of cuticle secreted by the ectoderm, though ingrowths from it may form a kind of internal skeleton for attachment of muscles. But Echinodermata and Vertebrata have an endoskeleton of mesoderm which is of great importance.
In simpler triploblastic animals, the mesenchyme forms a peculiar cellular tissue called parenchyma which forms a packing around all organs, and through which nourishment is conveyed to all parts from the alimentary canal, and gases and waste nitrogenous substances are conveyed to the excretory organs.
These relatively simple triploblastic animals have no body cavity or coelom, they are known as acoelomate animals (Platyhelminthes, Aschelminthes, Acanthocephala and Entoprocta).
In some higher triploblastic animals either the mesoderm becomes split into two layers, an outer parietal or somatic mesoderm and an inner visceral or splanchinic mesoderm, the space between the two layers of mesoderm is an extensive, fluid-filled, perivisceral coelom which is called a schizocoelous coelom.
In other triploblastic animals, pouches arise from the archenteron, they fuse together to form a coelom which is known as an enterocoelous coelom.
A coelom is found in all higher triploblastic animals which are grouped together as coelomate phyla (Chaetognatha, Pogonophora, Phoronida, Ectoprocta, Brachiopoda, Sipunculida, Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata, and Chordata). The coelom constitutes one or more perivisceral spaces around the heart, alimentary canal and other organs, it contains a coelomic fluid.
The internal organs of the coelom of triploblastic animals become large and are not affected by movements of the body wall, and they are able to freely perform movements of their own. In coelomate animals the gonads arise from the walls of the coelom, and germ cells are shed either into the perivisceral coelom, or the gonad itself contains a separated portion of the coelom.
The coelom communicates with the exterior by either dorsal pores (earthworms) or by two sets of ducts called nephridia and coelomoducts. Nephridia are intracellular ectodermal tubes which remove water and excretory matter.
Coelom ducts are mesodermal tubes which open usually at one end into the coelom and at the other end to the exterior, they may be only excretory or only for taking out germ cells or they may combine both the functions.
In some coelomates is a space containing blood and lymph, it is usually in the form of a branching system of tubes through which the fluid is made to circulate by a muscular heart, this space is a haemocoele. In some coelomates (Arthropoda and Mollusca) the haemocoele forms large perivisceral sinuses around the internal organs, but it never contains germ cells nor does it communicate with the exterior.
The enlarged haemocoele reduces the coelom to small cavities in the excretory and reproductive organs. In such animals the haemocoele is spoken of as a primary body cavity, while the coelom is called a secondary body cavity.
The body of the embryo or the adult of a triploblastic coelomate animal consists of a longitudinal series of more or less similar segments, such an animal is said to be metamerically segmented or to show metamerism. In metamerism there is a serial repetition of homologous parts which work in cooperation with the others for the, benefit of the body as a whole, the segments are integrated and interdependent.
In many coelomates, most of the organs are arranged mathematically throughout the length of the body which itself is divided into segments (Annelida), the muscles, glands, nephridia, ganglia, nerves, blood vessels and coelomic chambers are repeated in the segments.
The embryos of vertebrates show conspicuous metamerism, which is hidden in the adults by structural advances, so that metamerism is never uniform throughout the adult.
One of the factors which obscures metamerism is the specialisation of the anterior end to form a head, this is called cephalisation and it is due to a concentration of sense organs at the anterior end along with the formation of a brain. Formation of limbs and restriction of internal organs to certain segments also obscures metamerism.
On the basis of embryological development, the Metazoa are divided into two main evolutionary lines. One line contains the flatworms, annelids, molluscs, arthropods, and several smaller phyla, they constitute the division known as Protostomia (protostomes). From the other line have evolved the echinoderms, chordates and several smaller phyla, they are known as Deuterostomia (deuterostomes).
Each line displays a plan of development distinct from the other, though all members of each group do not have an identical pattern of development and there are many modifications in every phylum mainly due to the distribution and the amount of yolk present in the egg.
In protostomes, the mouth is formed usually from the blastopore, coelom is schizocoelic, and the fate of the blastomeres is fixed at a very early stage of development. If the egg of a marine annelid undergoes two cleavages to form four blastomeres, and these blastomeres are separated, then each will develop only into a fixed quarter of the gastrula and the larva.
Thus, each blastomere has a fixed and predetermined fate which cannot be changed even if the cell is moved from its original position.
This formation of blastomeres with fixed fates is known as determinate cleavage. Moreover, in protostomes cleavage is total and the axes of cleavage planes are oblique to the polar axis (the axis passing from the animal to the vegetal pole).
Such cleavage results in blastomeres having a spiral arrangement so that any single blastomere lies between two cells above or below it, and each tier of cells alternates with the next tier. Such a cleavage pattern is known as spiral cleavage. Thus, determinate and spiral cleavage are characteristic of protostomes.
In deuterostomes, the blastopore forms the anus, coelom is enterocoelic, and the fate of blastomeres is not fixed. If the egg of a starfish divides twice to form four blastomeres which are then separated, then each blastomere is capable of forming a complete gastrula and then a larva.
In the embryo of a frog the ectoderm cells of the mid-dorsal side give rise to the central nervous system. If, however, the ectoderm cells from the sides of the early gastrula are transplanted dorsally above the notochord, then these cells will form the central nervous system.
Thus, in deuterostomes the ultimate fate of blastomeres is not fully fixed and they can follow different lines of development, such formation of blastomeres with unfixed fates is known as indeterminate cleavage. Moreover, the pattern of cleavage is also different.
The areas of early cleavage planes are either parallel or at right angles to the polar axis, and the resulting blastomeres are situated directly above or below one another, such a cleavage pattern is called radial cleavage. Thus, indeterminate and radial cleavage are characteristic of deuterostomes.