In this article we will discuss about the body organisation and development of animals.
Animal kingdom is divided into two major groups on the basis of number of cells which constitute the body of animals. These groups are called Protozoa and Metazoa. The protozoans are unicellar, eukaryotic, and heterotrophic animals, perform all sorts of basic functions like a metazoan animal.
The unicellular grade animals are mostly smaller in size (microscopic) with a permeable limiting membrane which is related to a high surface area to volume ratio that maintains the shape and size of the animal. In their life cycle, no blastula or larval stage is seen.
Next to Protozoa, all the animals are multicellular and are grouped under Metazoa. The Metazoa can be divided into Parazoa, Mesozoa and Eumetazoa. The parazoans and mesozoans are the lowest grade of metazoans in which the multicellular is the only common feature.
The Parazoa includes a single phylum Porifera which includes all the sponges. They show a primitive grade of organization and possess many peculiar features for which they have placed under a separate group, the Parazoa (almost animals) or a separate side-lined metazoan group.
The peculiar features are:
(i) Cellular grade of body organization,
(ii) No definite tissues or organs,
(iii) Absence of true embryonic germ layers,
(iv) Presence of canal system, internally lined by choanocytes,
(v) Internal supporting skeleton, made of spongin (collagenous connective tissue fibres), and various types of calcareous and silicious spicules,
(vi) Totipotent cells and
(vii) No co-ordination in the activities and movement of cells due to lack of nervous system.
Mesenchyme is a primitive connective tissue usually, but not always of mesodermal origin. In sponges the mesenchyme is a non-cellular, proteinous jelly-like matrix which contains amoeboid cells and spicules. The mesenchyme is secreted from the choanocytes and occurs between the epidermis (pinacoderm) and gastrodermis (choanoderm). The mesenchyme in sponges is sometimes called mesohyl.
The phylum Placozoa includes only a single species (e.g., Trichoplax adherens) which is the most primitive and simplest metazoans. The mesozoans include two phyla, such as Orthonectida (e.g., Rhopalura) and Rhombozoa (e.g., Dicyema) and their body cells form two tissue layers which may correspond apparently to ectoderm and mesoderm but endoderm is absent.
They do not have any larval stage that may be compared with the gastrula stage of other metazoans. They have no basal lamina in the epithelium and lack the usual cell types.
In the early eumetazoan condition, the body of animals (e.g., phyla Cnidaria and Ctenophora) consists tissue grade of organisation and are diploblastic because the animals possess two embryonic germ layers- ectoderm (epidermis) and endoderm (gastrodermis).
The germ layers mean which are constituted by different tissues and give rise several organs that ultimately form an individual. The tissues are the combination of similar cells that perform a distinct function.
The metazoans have two types of tissues— epithelial and connective. The epithelial tissues have a well-defined shape with scanty intercellular substance. They are usually in contact with one another and rest on basal lamina (basement membrane), a structure-less or fibrillar structure composed of collagenous (animal fibrous protein) fibrous substances.
They are classified according to the shape of the cells. The epithelial tissue may be—(i) Squamous—These are thin, tile-like flat cells, serve primarily as a boundary layer, (ii) Cuboidal—cube-like cells with polygonal outer surface. It covers the outer and internal surfaces. Other functions of cubical tissue are secretion or have acquired excretory properties and (iii) Colummar—the tissues are tall with elongated nucleus and their function is absorption.
Connective tissues are characterized by groups of cells embedded in a fluid or gel-like extracellular matrix. In vertebrates they contain collagen fibres or yellow elastic fibres or both with a jelly-like matrix. This tissue is concerned with the binding of different parts, forms the framework of bone marrow, liver, lymph node and also assume a skeletal function.
In diploblastic condition (e.g., cnidarians and ctenophores), the ectoderm gives rise to epidermis, and the endoderm gives rise to the inner lining of the coelenteron or called gastro-vascular cavity. The mesoderm is absent and the space is filled by a non-cellular jelly-like matrix, called mesoglea present in some hydrozoans.
In other groups of cnidarians and ctenophores the mesenchyme, a thick gelatinous or fibrous layer containing cells, is present between epidermis and gastrodermis and derived from the embryonic ectoderm. The coelenteron or gastrovascular cavity functions in digestion, gas exchange and circulation. It has a single opening at one end, called mouth, and anus is absent. So mouth acts both ingestion and egestion.
Nervous system is absent in protozoans (protists) and sponges, but they respond to some extent to the external stimuli. The nervous system in cnidarians is very primitive type and it consists of a diffused network of un-polarised nerve cells (neurons) and their processes (neurites).
The phyla Cnidaria and Ctenophora are placed within a division of Animal Kingdom, called Radiata, for the presence of radial symmetry which is believed to be primary or primitive.
Next to diploblastic condition, rest of the metazoans (from Platyhelminthes to Chordata) are triploblastic, i.e., they possess three embryonic germ layers—the upper ectoderm, middle mesoderm and inner endoderm.
Gradually they have acquired morphological specialization with the division of the physiological functions, seen in different organs and associated systems. Ultimately the complexity is increased in the various systems.
In platyhelminthes, the different tissues are associated to form the different organs and the body represents the organ-grade of organisation, e.g. protonephridia. In higher groups (e.g., higher invertebrates and chordates) the organs are joined to form the different systems with specific function and represent the organ-system grade of organization.
The ectoderm gives rise to the outer protective epidermis and its derivatives, foregut and hindgut, major portion of the nervous system in invertebrates, sense organs and nephridia of invertebrates. The mesoderm gives rise to the connective tissue, dermis, coelom, septa, mesenteries, muscles, notochord, skeletons, blood corpuscles, coelomocytes, circulatory system, excretory system, and reproductive system.
The endoderm gives rise to the lining of the gut (mid) and its associated structures, thyroid, thymus, respiratory system and urinary bladder. Dermis is the lower layer of skin formed from the mesoderm and contains connective tissue, blood vessels, etc.
Skin is the outer covering of the animal containing epidermis (ectoderm) and underlying mesodermal dermis. Cuticle is a layer of non-living material which covers the epidermis. The material is organic and composed of an amino-polysaccharide complex, called chitin. The chitin is found in the hard exoskeleton of insects and crustaceans.
The development of blood vascular system is related to the development of septa and mesenteries that create barrier for continuous circulation of the coelomic fluid and in the absorption of the surrounding tissues.
Coelomic fluid generally transport O2 and nutrients which are distributed to the different parts of the body by diffusion. The small, flat bilaterians (Platyhelminthes) depend on the diffusion to get the O2 from the surrounding medium.
The diffusion takes place across the whole surface of the body. In longer animals where diffusion is slow and septa and mesenteries create barriers in the continuous flow of coelomic fluid, they distribute the fluid throughout the body by using another type of circulatory system, called blood vascular system.
It is constituted by channels and spaces in the connective tissue, and it transports blood to the different parts of the body by arteries.
In invertebrates the blood channels are either tubular channels or a cavity-like structure. The tubular channels are called vessels and cavity-like structures are called sinuses. In arthropods and molluscs, the connective tissue gaps become expanded to form a large, spacious blood- filled cavity, called haemocoel.
Respiration is most essential for the exchange of O2 and CO2. In many small animals, the gases are exchanged by the diffusion through the body surface. In larger animals the body surface is not sufficient to make up the required O2 gases. So ectodermal (invertebrates) and endodermal (vertebrates) respiratory organs, such as gills pulmonary sacs, and lungs have developed.
The gills and pulmonary sacs in invertebrates are covered by delicate ectodermal membranes adopted for absorption of O2 from the surrounding aquatic medium. These organs are richly supplied with blood vessels. The respiratory product CO2 passes out by the gills or lungs.
Chart showing the derivatives of germinal layers in Invertebrates:
