Cephalopods are active predators that use rapid movements to dart toward their prey, which they capture with several long tentacles. Squids and octopuses use beak-like jaws to bite their prey and then inject poison to immobilize the victim. A mantle covers the visceral mass, but the shell is reduced and internal in squids, missing in many octopuses, and exists externally only in chambered nautiluses. Fast movements by a squid occur when it contracts its mantle cavity and fires a stream of water through the excurrent siphon. By pointing the siphon in different directions, the squid can rapidly move in different directions. The foot of a cephalopod has been modified into the muscular siphon and parts of the tentacles and head. Cephalopods are the only molluscs with a closed circulatory system. They also have well-developed sense organs and a complex brain. The ancestors of octopuses and squid were probably shelled molluscs that took up a predatory lifestyle. Shelled cephalopods called ammonites were the dominant invertebrate predators of the seas for hundreds of millions of years until their disappearance in the mass extinctions at the end of the Cretaceous period. Most squid are less than 75 cm long. In 2003, a squid with a mantle 2.5 meters long was captured near Antarctica. The specimen was possibly a juvenile, only half the size of an adult. Large squid are thought to feed on large fish in the deep ocean, where sperm whales are their only natural predators. Concept 33.5 Annelids are segmented worms All annelids (?little rings?) have segmented bodies. They range in length from less than 1 mm to 3 m for the giant Australian earthworm. Annelids live in the sea, most freshwater habitats, and damp soil. The phylum Annelida is divided into three classes: Oligochaeta (earthworms), Polychaeta (polychaetes), and Hirudinea (leeches). Oligochaetes are named for their relatively sparse chaetae, or bristles made of chitin. This class of segmented worms includes the earthworms and a variety of aquatic species. Earthworms eat their way through soil, extracting nutrients as the soil passes through the alimentary canal. Undigested material is egested as castings. Earthworms till the soil, enriching it with their castings. Earthworms are cross-fertilizing hermaphrodites. Two earthworms exchange sperm and then separate. The received sperm are stored while a special organ, the clitellum, secretes a mucous cocoon. As the cocoon slides along the body, it picks up eggs and stored sperm and slides off the body into the soil.
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