BIO 002 INVERTEBRATE BIOLOGY: INTRODUCTION PART 1 Course Outline: General characteristics and classification of the following phyla. Description of the general features and life history of the types under each phyla (Protozoans, Coelenterate, Platyhelminthes, Nematodes, Annelids, Arthropods, Mollusca, Echinoderms). INTRODUCTION An animal is either a vertebrate or an Invertebrate. This categorization of animals is based on the presence or absence of the vertebral column, usually referred to as BACKBONE or spine in highly evolved animals. The animals which possess vertebral column are called VETEBRATES. Conversely, those animals, which do not possess the vertebral column are called INVERTEBRATES. General Characteristics of Invertebrates Invertebrate’s animals are found on earth in very large numbers with about 95 percent of all identified and described species of living organisms being invertebrates. Some of their general characteristics are as follows: 1. All invertebrate animals are cold-blooded 2. They vary in size, from microscopic e.g. Protozoans to others which are enormously large, e.g. some giant cephalopods. 3. Their shape is variable and they show different kinds of symmetry but a large number of them are also asymmetric as well. 4. Apart from Protozoans, other invertebrate phyla shows multicellularity. 5. They show different grades of body-organization; Protozoans and poriferans exhibit cellular grade of organization, but coelenterates exhibit the beginning of the tissue grade organization which is more pronounced in higher invertebrates. 6. The integument of invertebrates is simple. 7. The gut may be entirely absent, or partially formed or fully formed. However, if present, it is always dorsal in position. Likewise, if the heart is present, it is always dorsal in position. The nerve cord is ventral in position and solid in organization. 8. The Circulatory system, if present in invertebrates, may be open type, or closed type or combination of both. 9. They show diversified excretory mechanisms. Some takes place through the general body surface, or by specialized cells such as flame cells or structures like nephridia, Malphighian tubules, green glands, and kidneys. 10. They undergo different methods of reproduction such as fission, conjugation, parthenogenesis and gametic fusion. Difference between Invertebrates and Vertebrates S/N Invertebrate Vertebrate 1 Appendages if present are more There are never more than two pairs of than pairs appendages. 2 May have non-living exoskeleton There is always an endoskeleton, made of living tissues like cartilage and bones. 3 The Central nervous system The Central nervous system is in the form controls solid ganglia connected by of a hollow thick-walled tube nerve bones to form a chain. 4 The nerve cord lies ventrally to the The neutral tube lies dorsally to the digestive digestive tract. tract. 5 The respiratory organs usually The respiratory organs develop in relation to develop from the ectoderm. the wall of the pharynx and consist of gills or lungs, but gill clefts are present in the embryo of all vertebrates. 6 The circulatory system is usually The Circulatory system is a closed system not closed. with arteries, veins and capillaries. 7 The heart is always dorsal in The heart is always ventral and anterior. position. 8 Respiratory pigment haemoglobin, Haemoglobin is contained in the red blood if present, is dissolved in plasma. corpuscles. 9 Blood flowing through the arteries Blood circulates through a closed system of may pass freely into the body vessels-arteries, capillaries and veins. cavity 10 True tail is not found. Anus is A true tail which may be defined as the post terminal and opens at the posterior anal continuation of the body axis may be end of the body present 11 Skin consists of only one layer. Skin consists of two distinct layers, an outer epidermis overlying a thick dermis. 12 Eyes develop from the skin and not Eyes develop as outgrowths of the brain from the brain. Compound eyes are compound eyes are never found. often present. 13 Alimentary canal present or absent Alimentary canal always present. 14 No gill slits present. Gill Slits present at least in the embryos. 15 Reproduction is either sexual or Reproduction is only sexual. asexual. .