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Class Cephalopdoda These are considered to be the most sophisticated molluscs, and possibly the most intelligent invertebrates. There are about 660 species and a large size range, the Giant squid (Architeuthis sp.), can be over 20 m long. The cephalopod fossil record goes back to the Cambrian, and were similar to the nautilus (see below) extant today. They have a wide range of behaviours, a relatively large and complicated brain, and a great capacity for learning. All are predators. The foot has developed into a number of prehensile arms with suckers around the mouth, with one or two modified for sperm transfer(see mating below); and a muscular funnel in the mantle cavity. This funnel is used in movement when water is forcibly expelled through it, a form of jet propulsion. They have a radula and a pair of beak-like jaws. They also have an ink gland which releases a cloud of ink through the anus when the animal is alarmed. The ink cloud can act as a decoy to a predator allowing the cephalopod to escape. There are two Sub-classes, the Nautiloidea with six species, and Coleoidea which includes the squids, octopuses and cuttlefishes. All are marine.
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| There are six species of Nautiloidea; they have a snail-like shell which is divided by septa into gas-filled chambers, with only the last chamber being occupied by the animal, see Nautilus pompilus, right. The septa are perforated in the middle, and through this a cord called the siphuncle passes (see the diagram below) and secretes gas into the empty chambers. A fully grown adult may have as many as 30 chambers. It takes around 15 to 20 years to reach sexual maturity. The empty chambers are buoyancy aids. The mouth is surrounded by up to 90 tentacles or arms; all suckerless. The hood acts as a cover or operculum when the animal withdraws into the shell. The eyes and nervous system are relatively simple compared to the Coleoidea (see below). Nautilus pompilus is found in the Indo-Pacific region near the sea bed in waters up to 500 m deep. During the night it rises nearer the surface to feed. It is about 20 cm long when fully grown. It swims backwards at about the same speed as a person swimming leisurely. |
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The Coleoidea have no external shell. Cuttlefish have an internal calcareous shell, squids have a thin cartilaginous pen, see Loligo sp. the common squid below, and in octopods the shell is absent, see Eledone sp. bottom left. Octopuses tend to be solitary, but squid are often found in shoals making them a commercially viable species for fishermen to catch. Squid and cuttlefish have 10 arms - 8 short and 2 long. Octopods have 8 arms; all of the same length. Coleoidea have large, sensitive eyes which share many features with vertebrate eyes (see left), e.g. iris, cornea, lens focused by muscles and retina. The pupil is slit-shaped and the slit is aligned so that it is kept horizontal. The vertebrate eye arises from the development of the brain the cephalopod eye arises from the development of the skin. The eyeball of the giant squid is nearly 50 cm across. Mating
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Chromatophores On the left is Loligo sp. the common squid below, which grows to about 30 cm long |
On the right is Loligo opalescens showing the major internal organs. Loligo sp. feed on shrimp and fish, and cuttlefish feed mainly on shrimp and crabs.. |
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On the right is the vampire squid, Vampyroteuthis infernalis. It lives in tropical and sub-tropical oceans in water between 700 - 3 500 m deep. It has light emitting organs (photophores) on its body, eyes and tentacles. It is a very fast swimmer, and feeds on copepods, prawns and Cnidarians. Its Latin name translates as "vampire squid from hell"!
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The blue-ringed octopus (Hapalochlaena lunata) is just 10 cm long. It is a very pretty animal and swims in an elegant manner among the coral reefs in the Pacific and Indian oceans. However it has a venomous bite potent enough to kill an adult human in just 15 minutes. The giant Pacific octopus can be 5 m long and weight up to 50 kg. The female lays her thousands of eggs in a sealed up burrow or crevice and looks after them washing them with a stream of water and grooming them to keep them free of parasites. The eggs can take eight months to hatch depending on the water temperature, and during all this time she does not feed. Soon after the eggs have hatched she dies. Although her size is large her life span is usually less than four years. Left is Eledone sp., note that it has a single row of suckers, some octopuses have a double row.
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