Chapter 32 Essay Animals have different nutritional modes than plants and fungi. Animals are not able to make all of their organic molecules and therefore need to ingest them. Animals ingest either other living organisms or nonliving organic material in order to obtain the nutrients. Enzymes are used to digest the food after it has been ingested. Animals are multicellulareukaryotes. Unlike fungi and plants whouse cell walls for structural support, animals use structural proteins such as collagen. Collagen is most prevalent in extracellular matrices. There are three types of intercellular junctions in animals: tight junctions, desmosomes, and gap junctions. Most animals reproduce sexually. For most of the life cycle, the cell is in a diploid stage. Most species have a sperm and an egg, where the flagellated sperm moves to the egg to form a diploid zygote. The zygote then goes through cleavage, where mitotic cell divisions occur. Cleavage usually leads to a multicellular stage called a blastula, where the cell forms a hollow ball. After blastula comes gastrulation, when layers of embryonic tissue are produced, resulting in a gastrula. Many animal life cycles also include a larval stage. A larva is a sexually immature form of an animal that is morphologically different than the adult stage. Larvae then undergo metamorphosis in order to develop into an adult. Despite morphological differences, all eukaryotes have homeoboxes, sections of DNA that are common. Hoxgenes are important in developing embryos and expressing genes. 542-525 million years ago, there was a burst of evolutionary development known as the Cambrian Explosion. Strata that formed before the Cambrian explosion lack recognizable animal phyla, but in the strata of 542-525 million years ago, there is an abundance of extant phyla. The fossils found have proven to be distant relatives of today’s animals. There are multiple hypotheses about the cause of the Cambrian explosion. One is that newly...