Cephalopoda
There are approximately 650 to 700 extant species of cephalopods in two subclasses and five orders. Cephalopods are strictly marine and are found in all of the world's oceans.
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 arms, tentacles, and funnel are all derivatives of the foot. Members of the order Nautiloidea have more than 90 tentacles; members of orders Sepioidea and Teuthoidea have eight arms and two tentacles; and members of orders Ocotopoidea and Vampyromorpha have eight arms. The mantle surrounds the visceral sac and possesses strong muscles required for contraction of the cavity and respiration. An opening in the mantle cavity serves as an inhalant aperture, whereas the funnel serves as the exhalent aperture. All cephalopods have one pair of unciliated ctenidia within the mantle cavity, with the exception of Nautilus, which has two pairs of ctenidia. The movement of water over the ctenidia is controlled by muscular contractions of the funnel or mantle wall. An external shell is possessed only by the Nautiloidea. Sepioids and teuthiods have reduced inner shells, while ocotopoids and vampyromorphans lack shells altogether. A cephalopod is also characterized by a horny beak secreted by the walls of the buccal cavity, and a radula within the buccal cavity.
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 , which may contain a crop; 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.
Locomotion in cephalopods is accomplished mainly by jet propulsion. To close its mantle completely, a squid fits two cartilaginous ridges on the mantle wall into two cartilaginous grooves on the opposite funnel wall; contraction of circular muscles around the mantle cavity then forces water out the funnel. The funnel can be aimed, allowing the animal to change its direction. Locomotion in other cephalopods can be accomplished by other means. Octopoids can use their arms to "walk," and sepioids and teuthoids possess lateral fins that can propel the animal.
Cephalopods are gonochoric. A female typically possesses a single oviduct. A male produces spermatophores that it transfers to the female's genital pore by means of a specialized arm or tentacle. In some species, the specialized arm tip may be pinched off and left in the female's mantle cavity: this is known as the hectocotylus arm. Mating in some cephalopods includes courtship rituals that may consist of color changes, body movements, or combinations of both. Cephalopods exhibit spiral cleavage and are protostomous, but they have no larval stage: their development is direct. Octopods typically tend their eggs until hatching. Most cephalopods are semelparous.
With the exception of Nautilus, cephalopods contain pigment-rich cells in the epidermis surrounded by cells containing contractile fibers. These cells, called chromatophores, are responsible for the ability of the cephalopods to change color and patterns accurately and rapidly in response to danger or emotion. Chromatophores may also be under hormonal control. When the contractile fibers are stimulated, they contract and expose a larger amount of color.
Cephalopods possess well-developed nervous systems and complex sensory organs. The ganglia are large and close to each other, forming a large brain. Certain upper lobes within the brain serve as controls for memory and learning. Cephalopods also possess ganglia elsewhere within the mantle cavity linked to the brain by giant axons that are involved with muscular contraction. The eyes in Nautilus are primitive, but in other cephalopods are highly developed and resemble vertebrate eyes with a cornea, lens, retina, and iris. These eyes are capable of forming images and distinguishing colors.
Cephalopods are of considerable economic importance to humans. Many species of squid and octopus are eaten. 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.
It is believed that cephalopods evolved from an ancient group of gastropods. The fossil record of cephalopods is extensive, and more than 10,000 fossil species have been named. The Ammonoidea arose during the late Paleozoic and grew to large numbers during the Mesozoic. An ammonite had an external, coiled shell similar to that of Nautilus. Ammonites were very successful -- scientists have described 600 genera based on shell type - but became extinct at the end of the Mesozoic. Belemnoids, which also appeared in the Mesozoic, had internal shells. They are believed to be the precursors of modern-day squid and cuttlefish. However, the ancestors of these Recent animals may have been living before the belemnoids.
References:
Kaestner, A. 1967. Pages 394-424 in Invertebrate Zoology, vol. 1. Interscience Publishers, New York.
Kozloff, E. N. 1990. Pages 447-462 in Invertebrates. Saunders College Publishing, Philadelphia and other cities.
Meglitsch, P. A. 1967. Pages 546-563 in Invertebrate Zoology. Oxford University Press, London.
Wood, J. B. 2000. The cephalopod page. http://is.dal.ca/~ceph/TCP/
Wood, J. B., C. L. Day, P. G. Lee, R. K. O'Dor, and M. Vecchione. CephBase. http://www.cephbase.dal.ca/
Contributors
Kristen Wheeler (author), Daphne G. Fautin (author).
Glossary
- bilateral symmetry
-
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
- ectothermic
-
animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature