Polyplacophora And Cephalopoda Essay, Research Paper

Polyplacophora & Cephalopoda

–Polyplacophora–

The phylum Polyplacophora, meaning many plates , contains chitons, marine mollusks with oval bodies bearing eight dorsal plates.

Chitons live only in marine habitats and are commonly found in rocky areas of the intertidal zone along the coasts of almost every sea. They are stationary and cling to their rocky surfaces.

Chitons are supported by eight calcareous plates called valves. Depending on the species, these plates may be small or large, covering most of the dorsal surface. The valves are an adaptation that allows chitons to move and to press tightly against irregular surfaces. Each valve overlaps the one posterior to it. Chitons can curl into a ball to protect their soft bodies from predators or from abrasion when they lose their anchorage on rock surfaces in heavy surf. Contraction of the lateral longitudinal muscles cause the chiton to curl into its protective ball form. Also, the fleshy mantle girdle contains small scales and spicules or bristles for protection.

Longitudinal muscles extending along the dorsal aorta contract causing movement.

Food is ingested by the toothy radula. Digestion begins at the mouth where food is ingested and sent to the pharynx and then to the esophagus. From the esophagus food is sent to the stomach where salivary and esophageal glands secretive amylase that mixes with food, hydrolyzing starches to sugar. From the stomach, food is swept by cilia into the intestine. Indigestible food is expelled from the anus.

Chitons have a three-chambered heart lying posteriorly beneath the plates. Two lateral atria obtain hemolymph from efferent vessels. The efferent vessels drain the ctenidia and pump hemolymph into the ventricle. Hemolymph exits the heart through the dorsal aorta and flows to the various viscera while some hemolymph flows to the head. Hemolymph enters the hemocoel where it then goes to the afferent vessels. Afferent vessels carry hemolymph back to the ctenidia.

The chiton s nervous system is simple. The circumesophageal ring has four longitudinal cords, two of which control the foot. Aside from the circumesophageal ring the chiton has only one other sense organ. Aesthetes are light receptors that are located in the valve plates.

Gonoducts exit the gonad and send gametes to the genital openings in the mantle groove. Fertilization occurs externally and chiton zygotes develop into planktonic trochophore larvae.

–Cephalopoda–

The cephalopods, head-footed, are marine animals that have eight arms, some having eight arms and two tentacles, extending from their head.

All cephalopods live in marine habitats. The nautilus lives in the tropical Pacific ocean. Squids live in the deep waters of the North Atlantic. Octopi habit the bottom of the ocean.

Cephalopods are the most active of the phylum Mollusca. Using their siphon as a jet propulsion system, cephalopods expel water from it causing them to thrust themselves through the water. The water is expelled by strong muscles in the mantle and direction can be controlled by adjusting the siphon backward or forward. Fins on the side of the squid act as stabilizers for swimming. The species Loligo is the fastest invertebrate and can swim in bursts of up to 43 KPH.

Unlike any other mollusc, cephalopods actively pursue prey. Usually feeding on fish or crustaceans, the cephalopod will use jet propulsion to hunt the prey and its suction-cupped arms to capture it. After the prey has been captured the cephalopod will use its chitinous beak to tear at its dinner s flesh. Sometimes the food is showered with digestive enzymes before being ingested. The radula conveys food to the gut where it then digested. Strong muscles also move food by peristalsis. Digestion and nutrient absorption begins in the stomach and then is completed in the cecum. The digestive glands, sometimes called the liver and pancreas, sends digestive enzymes to the stomach and cecum. The waste is sent to the rectum located near the base of the siphon.

Ctenidia, or gills, contain many highly vascularized ciliated filaments supported by a stiff chitinous rod. Muscular contraction draws water over the filaments where gas exchange occurs.

Most molluscs divide by spiral cleavage, however cephalopods develop by bilateral cleavage. Cephalopods do not have larval stages.

Cephalopods have the most specialized brain of all molluscs. It is protected by a cartilaginous cranium and contains large cerebral ganglia. The cerebral ganglia are attached to the optic lobes and optic nerves that serve the eyes. Beneath the cerebral ganglia is the pedal ganglia, which controls the arms, tentacles, and the funnel through a special system of giant nerve fibers. Theses giant nerve fibers are responsible for the fast attack and the escape movements. The visceral ganglia connects to the mantle and visceral nerves. The mantle nerves extend to the stellate ganglia which control the jet propulsion muscles. The cephalopod s eye very closely resembles a vertebrate s. It contains a lens, a cornea, a retina, and an iris. The eye muscles focus light much like a human s eye does, however the cephalopod s eye is a direct retina, meaning photoreceptive cells face forward and receive light directly.

The gonads are located in the posterior end of the cephalopod. Female cephalopods have one to two short oviducts located by a pair of nidamental glands which secrete albumen, yolk, and other protective membranes around the eggs. Male cephalopods have a large testis that sends sperm to the seminal vesicle by way of a coiled sperm duct. Sperm is stored in a spermatophoric sac near the funnel until copulation. Before copulation squids may swim acrobatically around each other and then they will position themselves head to head. The male wraps his tentacles around the female and uses his hectocotylus arm to reach into his funnel and pull out spermatophores. The male then places the spermatophores into the female s mantle where sperm are released. The female pulls out egg sacs after fertilization and places them in between rock crevices.