Sponges consist of an outer layer of flattened cells and an inner layer of cells called choanocytes separated by a jelly-like substance called mesohyl. However, sponges exhibit a diversity of body forms, which vary in the size and branching of the spongocoel, the number of osculi, and where the cells that filter food from the water are located. Water flows out through a large opening called the osculum (Figure 2). Water enters the spongocoel from numerous pores in the body wall. The body of the simplest sponges takes the shape of a cylinder with a large central cavity, the spongocoel. Figure 1: Sponges are members of the phylum Porifera, which contains the simplest animals. Much of the body structure of the sponge is dedicated to moving water through the body so it can filter out food, absorb dissolved oxygen, and eliminate wastes. Sponges live in intimate contact with water, which plays a role in their feeding, gas exchange, and excretion. All sponges are aquatic and the majority of species are marine. SpongesĪnimals in subkingdom Parazoa represent the simplest animals and include the sponges, or phylum Porifera (Figure 1). The cnidarians, or the jellyfish and their kin, are the simplest animal group that displays true tissues, although they possess only two tissue layers. Sponges are similar to what might have been the ancestor of animals: colonial, flagellated protists.

Although they have specialized cells for particular functions, they lack true tissues in which specialized cells are organized into functional groups.

Sponges appear to represent an early stage of multicellularity in the animal clade. The sponges and the cnidarians represent the simplest of animals. Invertebrates include a huge diversity of animals, millions of species in about 32 phyla, which we can just begin to touch on here. Although in general we are most familiar with vertebrate animals, the vast majority of animal species, about 95 percent, are invertebrates. The kingdom of animals is informally divided into invertebrate animals, those without a backbone, and vertebrate animals, those with a backbone. Describe the organizational features of cnidarians.Describe the organizational features of the simplest animals.By the end of this section, you will be able to:

How do sponges move