Zoology is one of the popular courses offered to students in science. Zoology is the branch of Biology dedicated to all aspects of animal life – from anatomy and physiology, to evolution and husbandry. It is a subject for those who love animals and those who want to understand how the natural world works. As our awareness of environmental issues grows, and as our governments shift their priorities towards protecting and conserving the environment and wildlife, so will Zoology be an increasingly worthwhile subject to study. Invertebrate zoology is the subdiscipline of zoology that consists of the study of invertebrates, animals without a backbone (a structure which is found only in fish, amphibians, reptiles, birds and mammals). Animals come in a bewildering diversity of forms and pursue an amazing breadth of life styles. Invertebrates are a vast and very diverse group of animals that includes sponges, echinoderms, tunicates, numerous different phyla of worms, molluscs, arthropods and many additional phyla. Zoology or Animal Biology is that branch of science which deals with animals. About half a million different sorts or species of animals have already been described and named, each breeding true to its own special characteristics, each different from all others. The number of fresh species discovered, described, and named every year is about two hundred, and this number is at present increasing, not decreasing, year by year. This textbook has been designed to meet the needs of B.Sc. (Hons.) Students of Zoology as per the UGC Choice Based Credit System (CBCS).