Context: Have you ever been to the ocean or eaten seafood? If you have, then youve probably encountered members of Phylum Mollusca. In addition to snails, mollusks include squids, slugs, scallops, and clams. You can see a clam in Figure 12.15. There are more than 100,000 known species of mollusks. Some mollusks are nearly microscopic. The largest mollusk, the colossal squid, may be as long as a school bus and weigh over half a ton! Watch this short video to see an amazing diversity of mollusks: . MEDIA Click image to the left or use the URL below. URL: Mollusks have a true coelom and complete digestive system. They also have circulatory and excretory systems. They have a heart that pumps blood, and organs that filter out wastes from the blood. You can see some other traits of mollusks in the garden snail in Figure 12.16. Like the snail, many other mollusks have a hard outer shell. It is secreted by special tissue called mantle on the outer surface of the body. The shell covers the top of the body and encloses the internal organs. Most mollusks have a distinct head region. The head may have tentacles for sensing the environment and grasping food. Mollusks generally have a muscular foot, which may be used for walking or other purposes. A unique feature of mollusks is the radula. This is a feeding organ with teeth made of chitin. It is located in front of the mouth in the head region. It can be used to scrape algae off rocks or drill holes in the shells of prey. You can see the radula of the sea slug in Figure 12.17. Mollusks reproduce sexually. Most species have separate male and female sexes. Fertilization may be internal or external, depending on the species. Fertilized eggs develop into larvae. There may be one or more larval stages. Each one is different from the adult stage. Mollusks live in most terrestrial, freshwater, and marine habitats. However, the majority of species live in the ocean. They can be found in both shallow and deep water and from tropical to polar latitudes. They have a variety of ways of getting food. Some are free-living heterotrophs. Others are internal parasites. Mollusks are also eaten by many other organisms, including humans. Annelids are segmented worms in Phylum Annelida. There are about 15,000 species of annelids. They range in length from less than a millimeter to more than 3 meters. To learn more about the amazing diversity and adaptations of annelids, watch this excellent video: [Link] MEDIA Click image to the left or use the URL below. URL: Annelids are divided into many repeating segments. The earthworm in Figure 12.18 is an annelid. You can clearly see its many segments. Segmentation of annelids is highly adaptive. Each segment has its own nerve and muscle tissues. This allows the animal to move very efficiently. Some segments can also be specialized to carry out particular functions. They may have special structures on them. For example, they might have tentacles for sensing or feeding, paddles for swimming, or suckers for clinging to surfaces. Annelids have a large coelom. They also have several organ systems. These include a: circulatory system; excretory system; complete digestive system; and nervous system, with a brain and sensory organs. Most annelids can reproduce both asexually and sexually. Asexual reproduction may occur by budding or fission. Sexual reproduction varies by species. Some species go through a larval stage before developing into adults. Other species grow to adult size without going through a larval stage. Annelids live in a diversity of freshwater, salt-water, and terrestrial habitats. They vary in what they eat and how they get their food. Some annelids, such as earthworms, eat soil and extract organic material from it. Annelids called leeches are either predators or parasites. Some leeches capture and eat other invertebrates. Others feed off the blood of vertebrate hosts. Annelids called polychaete worms live on the ocean floor. They may be filter feeders, predators, or scavengers. The amazing feather duster worm in Figure 12.19 is a polychaete that has a fan-like crown of 
 Question: ___special tissue on the outer surface of a mollusk that secretes a shell
Output: mantle