UNIT 3 METAZOA - ORIGIN AND EVOLUTION Structure 3.1 Introduction Objectives 3.2 Levels of Body Organisation 3.3 Characteristics of Metazoa 3.4 Symmetry Asymmetrical and Spherical Radial and Biradial I Bilateral 3.5 Developmental Patterns Cleavage Fate of Blastopore 3.6 Germ Layers 3.7 Body Cavity and Coelom Pseudocoelom Coelom 3.8 Cephalisation and Segmentation 3.9 Origin and Evolution of Metazoa Syncytial Theory Colonial Theory Polyphyletic Theory Evolution of Metazoa 3.10 Summary 3.11 Terminal Questions 3.12 Answers 3.1 INTRODUCTION You have already seen in Unit-1 that in the two kingdom classification, the unicellular 'animals' used to be clubbed together under a single phylum Protozoa that constituted sub-kingdom - Protozoa. The rest of the animals, all multicellular, were grouped under the sub-kingdom Metazoa under various phyla (the corresponding grouping for plants was Protophyta and Metaphyta). However, under the present concept of Flve Kingdom Classification, this grouping has no relevance. Still, we often continue to use the term Metazoa to refer to the Animalia of the five kingdom classification. In th~sUn~t we start with an explanation of the levels of body organisation in animals and the baslc animal bodjr plan. However, diverse the different invertebrates and vertebrates may appear to the eye, it is possible to group them in four master body plans. These are the unicellular plan, the cell aggregate plan, blind sac plan and tube within a tube plan. The protozoans fall into the first category and the rest three structural plans are seen in the metazoans. We next list out the characteristic features of metazoans. We shall also discuss those features that are considered of fundamental importance for describing and understanding the structure and classification of any animal. These characters are: (i) cleavage patterns and number of genn layers it has been derived from; (ii) its body symmetry; (iii) nature of body cavity; (iv) segmentation and cephalisation,. It would be useful if you rev~se Block-3 of the course Developmental Biology (LSE-06) before reading this unit as most of the concepts discussed there would help you to understand this Unit better. In later sections of the unit we will consider the variou? theories related to origln and evolution of metazoans or the Animalia. Objectives After studying this unit you should be able to: describe various levels of body organisation of animals; e describe the various cleavage patterns found in animals; identify the various types of gem layers and describe the functions of their ~ctazor- mgin and ~volution derivatives; identify the animal groups based on their symmetry;. describe the different types of body cavities, segmentation, cephalisation and theu functional significance; classify animals on the basis of their structural organisation; discuss the origin and evolution of Animalia. 3.2 LEVELS OF BODY .ORGANISATION You have learnt in your earlier course LSE-01 about the levels of organisation of matter. You may recall that the smallest structural units of all matter are subatomic~particles, mainly electrons, protons and neutrons. The next larger units are atoms and many atoms get together to form combinations called compounds which are variously joined together to give a higher level of organisation called complexes of compounds. Such levels of matter can be viewed as a pyramid (Fig. 3.1). Chemical / compounds \ / Atoms \ Sub atomic particles Fig. 3.1: Levels of organisation of matter. b In this pyramid any given level contains all lower, levels as its,component and itself is 1 also a component of all higher levels. For example, atoms contain subatomic particles as components, and atoms are themselves components of chemical compounds. Similarly, in ;I living matter, complexes of compounds occur as submicroscopic and mic~oscopicbodies 4 called organelles, capable of carrying on specialised functions within the cell. Organisms which are made of just one cell are the simplest and the most primitive creatures called unicellular organisms. Their level of body organisation is at the lowest and is called protoplasmic level of body organisation. If we try and fit the metazoan body organisation into a pyramid mode, the protoplasmic level will be at the bottom. As the organisms evolved from unicellular to multicellular grade their level of body orianisation also changed from simple to complex. Fig. 3.2: Level of body organlaation. Look at the Figure 3.2, the next higher level of organisation is the cellular level. This is really an aggregation of cells that are functionally differentiated. A division of labour is evident so that some cells are specialised for reproduction some for nutrition. Among the metazoans, Placozoa and Mesozoa are.said to belong to the cellular level of body I organisation (Fig. 3.3 e and h). Diversity of Animal Life-I some authorities place the sponges (porifera) among this group too because they have (Organisation) several cell types differentiated for various fu'nctions but there is no true tissue orpanisation yet. (Fig. 3.3 c). The phylum Placozoa contains a single species of a minute marine animal Trichoplar adharens composed of a dorsal and ventral epithelial layer enclosing loose mesenchyme like cells. Mesozoans comprise some 50 species of small parasitic worms that have simple structures made up of 20-30 ciliated cells covering a few reproductive cells. Fig. 3.3: (a) Cellular level of body organisation in mesozoan - Rhopcllura. (after Hyman) (b) a placozoan (after Margurlis & Schwartz 1982) (c) L.S. asconoid sponge. As you already know a tissue is a group of cells similar in origin and structure that perform a specific function. The next level, is the tissue level of body organisation which can be seen in coelenterates (Cnidaria and Ctenophora). These are made up of two germ layers ectoderm forming epidermis and endoderm forming the gastrodemis. The jelly fishes and their relatives are considered as the beginning of tissue organisation - and an excellent example of tissues, in Cnidarians is the nerve net in which the nerve cells and their processes form a definite tissue structure with the function of coordination (Fig. 3.4). The next higher level of body organisation as seen in the pyramid is the organ. Organs are usually made up of more than one kind of tissues. This is already seen in some cnidarians, ctenophores and the flatworms or Platyhelminthes in which there are well defined organs such as the eye and reproductive organs (Fig. 3.5). Fig. 3.4: Tissue grade of organisation - nerve net in jelly fish. lenlacular shcalh - aboral canal Fig. 3.5: Organ level of organisation in a typical ctenophore. When organs work together to perform a specific function we have the highest level of body organisation i.e. the organ system level of body organisation. The systems are associated with basic body functions. This type of body organisation is seen for the first time in Platyhelminthes (Fig. 3.6) which have for example, a digestive system distinct from a well developed reproductive system. From this phylum to mammals, all animals have the highest level of body organisation. Metazoa - Origin and Evolutiorr Fig. 3.6: Organ system level of organisation in Plan& a) digestive system b) excretory systenl c) reproductive system. -- 3.3 CHARACTERISTICS OF METAZOA From Unit-2 you know that the unicellular protozoans are highly versatile and successful organisms that show remarkable organisation and division of labour within the confines of the single cell. This diversity is achieved by varying the structure of their organelles at the sub-cellular level. The Metazoa or the multicellular animals have achieved their structural diversity by varying lheir cells that have become specialised to perform different functions. These cells are normally incapable of independent existence. Let us list out some of the features that characterise metazoans. 1. Members of Metazoa possess a complex multicellular structural organization which may include the presence of tissues, organs and organ systems. 2. In the life history of metazoans, typically a fertilized egg passes through a blastula stage in the course of its early embryonic development before changing into an . adult. 3. Since metazoans are multicellular they are relatively larger in size than unicellular protozoans. Naturally, their ntitritional requirements are more and they have to search for food. Consequently, locomotion in metazoans is highly developed and for this purpose they have evolved contractile muscular elements and nervous structures. 4. The ability for locomotions has influenced the shape of the metazoan animals which in turn has conferred specific types of symmetries to metazoan groups. 5. Most of the metazoans show differentiation of the anterior end or head (cephalization); associated with cephalisation, there is the centralization of the nervous system in the head region. Although all metazoans share some characteristic features, their body plans differ in symmetry, internal organisation, developmental patterns and modes of formation of,body cavity. These differences provide us a means of grouping them or organising them into different phyla. Let 3s discuss these features one by 'one. All living organisms have some body shape and form. The general body plan of animals may be organized in one of several ways (Fig. 3.7 a-f). 59 Diversity of Animal Life-I IOrganisation) (b) actinophrys tentacular Ib p haryn y eal plane Flg. 3.7: Different types of body symmetries a) asymmetrical b) spherically symmetrical, (c-d) radially symmetrical. (e) biradially symmetrical (f) bilaterally symmetrical. Arrangement of parts or organs on either side of an imaginary dividing line or around a common axis or radially around a point so that opposite parts are mirror images of one another is called symmetry. There are two broad divisions of symmetry, (i) primary, or embryonic (ii) secondary, or adult. .The latter may or may not be the same as the primary one.