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Most of the more than one million species of multicellular animals known to exist on Earth are invertebrates, or animals without backbones. A variety of otherwise unrelated groups of animals are lumped together by scientists as invertebrates. The only trait that distinguishes invertebrates from vertebrates, or animals with backbones, is the absence or presence of a backbone.
More than two dozen phyla (plural of phylum) of living invertebrates, plus many extinct forms, are recognized. Some of the largest and most important phyla of invertebrates are Sponges; Jellyfishes and sea anemones; Flatworms, Rotifers, horsehair worms, and roundworms; Mollusks (snails, bivalves, squids, and octopuses), Annelida (segmented worms), Arthropoda (horseshoe crabs, spiders, crabs, centipedes, millipedes, and insects), and Echinodermata (starfishes and sea urchins).
The large group of animals called mollusks live on land and in both fresh and salt water. They constitute the phylum Mollusca, a major group of animals known to have as many as 100,000 living species and more than 50,000 fossil forms. Most mollusks, including snails, clams, oysters, and mussels, have shells. A major group called the cephalopods, however, including octopuses, squids, and cuttlefish, have shells that are either greatly diminished or absent. One of the most distinctive anatomical features characteristic of mollusks is a true coelom, a body cavity that contains most of the vital organs. The digestive tract, heart, liver, and reproductive organs are all housed inside the coelom. Many of the lower invertebrates, in contrast, have no coelom.
Two other features that are characteristic of the mollusks and absent in most other groups of animals are the visceral mass and the mantle. The visceral mass is the main body of the animal and contains all the vital organs. The name “mollusk”, which is derived from a Latin word meaning “soft,” refers to this large, soft body mass. The mantle is a thick covering of tissue that surrounds the visceral mass and has glands that secrete the shell, if the animal has one. Many of the aquatic mollusks also have another distinctive feature—gills that are enclosed within a cavity formed by the mantle.
Cephalopoda is the most morphologically and behaviorally complex class in phylum Mollusca. Cephalopoda means "head foot" and this group has the most complex brain of any invertebrate. Cephalopods are characterized by a completely merged head and foot, with a ring of arms and/or tentacles surrounding the head. The mantle surrounds the visceral sac and possesses strong muscles required for contraction of the cavity and respiration.
All cephalopods are carnivorous. The strong beak is at the entrance to the buccal cavity , on the floor of which lies the radula. There are two pairs of salivary glands , one of which may be poisonous. The digestive tract consists of three parts: esophagus; stomach , which mashes food; and caecum , where most digestion and absorption occur. The posterior portion of the caecum contains a diverticulum that serves as an ink gland, producing a suspension of melanin that can be expelled through the mantle cavity.
Cephalopods are of considerable economic importance to humans. Many species of squid and octopus are edible. Nautilus shells are often used decoratively, and the internal shell of a cuttlefish, or cuttle bone, is sold in the pet trade as a calcium source for birds. Giant cephalopods such as squid and octopuses are also a great source of sea-monster folklore.
Cephalopods are the most intelligent, most mobile, and the largest of all molluscs. Squid, octopuses, cuttlefish, the chambered nautilus, and their relatives display remarkable diversity in size and lifestyle with adaptations for predation, locomotion, disguise, and communication. Today, biologists and paleontologists continue to captivate the human mind and imagination with details of these molluscs' behavior, natural history, and evolution.
The structure of cephalopod eye is probably the most sophisticated eye of all invertebrates and is as complex as the vertebrate eye, though the two are not homologous. For their body size, cephalopod eyes are relatively large. They contain an iris, pupil, and lens, but not necessarily a cornea. Octopuses are the only cephalopods with a completely protected "closed" cornea. That means that the eyes of squids and cuttlefish are in direct contact with sea water! The pupil in cephalopods is unique in that its morphology is different in octopuses, cuttlefish, and squid. Octopuses have a slit-shaped rectangular pupil. In cuttlefish it is W-shaped, and in squid it is round.
Arms and tentacles are another distinguishing cephalopod characteristic. All cephalopods have arms, but not all cephalopods have tentacles. Octopuses, cuttlefish, and squid have eight non-retractable arms, but only cuttlefish and squid have tentacles (two each). Arms usually have palps, often suckers, and sometimes hooks along their undersides. These can be attached to the arm directly or by a flexible stalk and are used to adhere to substrates and catch prey. Tentacles are longer than arms, are retractable, and usually have a blade-shaped or flattened tip, which is covered in suckers.
Cephalopods have an amazing ability to change color very rapidly. They accomplish this feat using numerous pigment-filled bags, called chromatophores. Chromatophores are found in the skin, and expand and contract to reveal or conceal small dots of color. They can be so densely concentrated that 200 may be found in a patch of skin the size of a pencil eraser.