<P> A phylum is the highest level in the Linnaean system for classifying organisms . Phyla can be thought of as groupings of animals based on general body plan . Despite the seemingly different external appearances of organisms, they are classified into phyla based on their internal and developmental organizations . For example, despite their obvious differences, spiders and barnacles both belong to the phylum Arthropoda, but earthworms and tapeworms, although similar in shape, belong to different phyla . As chemical and genetic testing becomes more accurate, previously hypothesised phyla are often entirely reworked . </P> <P> A phylum is not a fundamental division of nature, such as the difference between electrons and protons . It is simply a very high - level grouping in a classification system created to describe all currently living organisms . This system is imperfect, even for modern animals: different books quote different numbers of phyla, mainly because they disagree about the classification of a huge number of worm - like species . As it is based on living organisms, it accommodates extinct organisms poorly, if at all . </P> <P> The concept of stem groups was introduced to cover evolutionary "aunts" and "cousins" of living groups, and have been hypothesized based on this scientific theory . A crown group is a group of closely related living animals plus their last common ancestor plus all its descendants . A stem group is a set of offshoots from the lineage at a point earlier than the last common ancestor of the crown group; it is a relative concept, for example tardigrades are living animals that form a crown group in their own right, but Budd (1996) regarded them as also being a stem group relative to the arthropods . </P> <P> The term Triploblastic means consisting of three layers, which are formed in the embryo, quite early in the animal's development from a single - celled egg to a larva or juvenile form . The innermost layer forms the digestive tract (gut); the outermost forms skin; and the middle one forms muscles and all the internal organs except the digestive system . Most types of living animal are triploblastic--the best - known exceptions are Porifera (sponges) and Cnidaria (jellyfish, sea anemones, etc .). </P>

What is the thought to have caused the cambrian explosion