In several respects, sponges are the simplest animals. For this reason, they were long thought to be the sister group of the remaining animals, until genome sequencing placed the ctenophores in that position. But some of the perceived complexity of ctenophores now appears to have evolved independently from other animal lineages, whereas sponges have retained more of the features of the ancestral animal. Although sponges have some specialized cells, they have no distinct embryonic cell layers and no true organs. Early naturalists classified sponges as plants because they were sessile and lacked body symmetry.

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Sponges have hard skeletal elements called spicules, which may be small and simple or large and complex (see Figure 30.2B). Three major groups of sponges, which separated soon after the origin of sponges, are distinguished by their spicules. Members of two groups (glass sponges and demosponges) have skeletons composed of silicaceous spicules made of hydrated silicon dioxide (Figure 30.17A and B). These spicules are remarkable in having greater flexibility and toughness than synthetic glass rods of similar length. Members of the third group, the calcareous sponges, take their name from their calcium carbonate spicules (Figure 30.17C). There is some question about the monophyly of sponges, although genomic analyses that combine information from many genes support their monophyly.

The body plan of sponges of all three groups—even large ones, which may reach 1 meter or more in length—is an aggregation of cells built around a water canal system. Sponges bring water into their bodies by beating the flagella of their specialized feeding cells, called choanocytes (see Figure 30.2B). Water, along with any food particles it contains, enters the sponge by way of small pores and passes into the water canals or a central atrium, where the choanocytes capture food particles. (You may recall from Key Concept 30.1 that the choanocytes are similar in structure to protists known as choanoflagellates, which may indicate a retained ancestral similarity between the two groups.)

A skeleton of simple or branching spicules, often combined with a complex network of elastic fibers, supports the body of most sponges. Sponges also produce an extracellular matrix, composed of collagen, adhesive glycoproteins, and other molecules, that holds the cells together. Most species are filter feeders; a few species are predators that trap prey on hook-shaped spicules that protrude from the body surface.

Most of the 8,500 species of sponges are marine animals; only about 50 species live in fresh water. Sponges come in a wide variety of sizes and shapes that are adapted to different movement patterns of water. Sponges living in intertidal or shallow subtidal environments with strong wave action are firmly attached to the substrate. Most sponges that live in slowly flowing water are flattened and are oriented at right angles to the direction of current flow. They intercept water and the food items it contains as it flows past them.

Sponges reproduce both sexually and asexually. In most species, a single individual produces both eggs and sperm, but individuals do not self-fertilize. Water currents carry sperm from one individual to another. Sponges also reproduce asexually by budding and fragmentation.