Paul A. Meglitsh
Squid, any of a large group of marine mollusks that have a greatly reduced shell, a cylindrical body, and a well-developed head to which ten arms are attached. Two of these arms have long tentacles bearing suction cups near their tips. Below the head, the squid body is covered by a conical muscular mantle that is equipped with lateral fins. The characteristic mollusk shell typically consists only of a horny "pen" or gladius entirely embedded in the mantle. Squids vary in size from about 2 inches (5 cm) to the giant squid Architeuthis, which reaches a length of up to 20 feet (6 meters) and is the largest of all invertebrates. They are widely distributed throughout the world, near the surface and at great depths.
Squids are classified in the order Teuthoidea, class Cephalopoda, phylum Mollusca. They can be distinguished from octupuses by their ten arms, more elongate form, and pelagic habits, and from cuttlefishes and other ten-armed mollusks by their greatly reduced shell.
Circulatory, Excretory, and Nervous Systems
Squids have a complex circulatory system with an extensive set of arteries and veins and relatively few open lacunar spaces. Flowing to the heart, the blood passes through capillaries in a pair of large gills located in the mantle cavity. Here, oxygen is absorbed with the aid of a respiratory pigment, hemocyanin. Two kidneys, closely associated with the pericardial cavity and circulatory system, open through excretory pores into the mantle cavity. Ammonia is the chief excretory product.
Squids have a large brain protected by cartilagelike shields in the head. The brain is formed by the concentration of ganglia, which are separate in most other mollusks. The brain is complex, containing a number of definite centers for the control of specific body parts or activities and a relatively well-developed associational center. An interesting feature of the nervous system is a set of giant neurons that provide fast coordination of the mantle musculature and arms. The huge nerve cells of the squid have been popular subjects for neurophysiological research. The excellent sense organs and complex brain of the squid are reflected in rather complex behavior patterns.Internal Anatomy of a Squid
Water enters the squid's mantle cavity through spaces between the mantle and head. When the mantle is contracted, these spaces are closed. Water then is ejected forcibly through a siphon, propelling the squid through the water. Many squids are powerful swimmers, capable of capturing fish and other aquatic animals with ease. The strongest swimmers are the so-called flying squids (family Onycoteuthidae), which have a slender body, relatively short arms, and large terminal fins. They achieve high speeds and are able to leap 10 feet (3 meters) or more above the surface of the sea.
Feeding and Digestion
Squids grasp their prey with the suction cups on their arms, bring it to the mouth, and attack it with their strong, horny jaws. The food is further fragmented by the radula, a toothed, tonguelike organ in the mouth cavity. The stomach opens into the intestine and a cecum. Skeletal remains and larger hard parts pass directly into the intestine, while smaller fragments enter the cecum. Flat, leaflike plates in the cecum serve as sorting surfaces. Ciliary currents carry rejected particles into the intestine. Food is digested and absorbed in the cecum and perhaps in the digestive gland.
Ink glands, forming a black or (in some deep-sea squids) a luminescent fluid, are located near the anus. When attacked, the squid ejects this ink to confuse the predator.
In squids the sexes are separate. Males store sperm in intricate spermatophores. One of the male's arms is modified into a hectocotylus, which the male uses to pluck out stored spermatophores and insert them into the female mantle cavity near the gonopore. An ejaculatory organ in the spermatophore pushes the sperm into the female's mantle cavity. The eggs, which are fertilized internally, are surrounded by an albuminous mass that hardens when it is later extruded and touches seawater. Many squids deposit their eggs singly, but Loligo produces egg strings. The young squids hatch at a relatively advanced stage, looking much like diminutive adults.
Squids are prized as seafood in the Mediterranean area and on the coasts of Asia, where they are of considerable economic importance. Squid are also an important part of the marine food chain. Giant squids, for example, are important food organisms for sperm whales.