CLASS CEPHALOPODA (L. cephalic, head + Or. podos, foot)
The class Cephalopoda includes the octopuses. squid, cuttlefish. and nautili. They are the most complex molluscs.
- The anterior portion of their foot is modified into a circle of tentacles or arms. It is used for prey capture, attachment, locomotion, and copulation.
- The foot with mantle cavity is modified into funnel. This funnel is used for jet like locomotion.
- Their head is in line with the visceral mass.
- Cephalopods have a highly muscular mantle. Mantle encloses all of the body except the head and tentacles. The mantle acts as a pump to bring large quantities of water into the mantle cavity.
Different types of shell are present in cephalopods:
1. External shell: The ancestral cephalopods had a conical shell. The only living cephalopod that possesses an external shell is the nautilus. Septa subdivide its coiled shell. As the nautilus grows, it moves forward. It secrets a new shell around itself and leaves an empty septum behind. Only the last chamber is occupied. These chambers are fluid filled. A cord of tissue called a siphuncle perforates the septa. It absorbs fluids by osmosis and replaces this fluid with metabolic gases. The amount of gas in the chambers is regulated. Therefore, it changes the buoyancy of the animal.
2. Reduced internal shell: The shell is reduced or absent in all other cephalopods. In cuttlefish. the shell is internal. It is laid down in thin layers. Therefore, it leaves small, as-filled spaces. It increases buoyancy. Cuttlefish shell is called cuttlebone. It is used to make powder for polishing. It is also fed to pet birds to supplement their diet with calcium.
3. Pen: The shell of a squid is reduced. It has internal. chitinous structure called the pen. quid also have cartilaginous plates in the mantle wall, neck, and head. These plates upport the mantle and protect the brain.
4. Without shell: The shell is absent in octopuses.
Cephalopods are predators. Therefore, cephalopods depend on their ability to move quickly. They move by jet-propulsion system. The mantle of cephalopods contains radial and circular muscles. The circular muscles contract. It decreases the volume of the mantle cavity. It also closes collar like valves. This closing of valve prevents water from moving out of the mantle cavity between the head and the mantle wall. Water is thus forced out of a narrow funnel. Muscles attached to the funnel control the direction of the animal’s movement. Radial muscles of mantle increase the volume of cavity. It brings water into the mantle cavity. Posterior fins act as stabilizers in squid. It also helps in propulsion and steering in cuttlefish. Octopuses are more sedentary animals. They may use jet propulsion to escape. It normally crawls over the substrate using their tentacles.
In most cephalopods, the use of the mantle in locomotion takes place at same time With the loss of an external shell. Therefore, a rigid external shell comes before the jet-propulsion method.
FEEDING AND DIGESTION
Cephalopods legate their prey by sight. They capture prey with tentacles. The tentacles have adhesive cups. In squid. the margins of cups have tough protein. They sometimes possess small hooks. All cephalopods have jaws and a radula. The jaws are powerful, beak like structures. They are used for tearing food. The radula rasps food and forces it into the mouth cavity.
Cuttlefish and nautili feed on small invertebrates on the ocean floor. Octopuses are nocturnal hunters. They feed on fish and crustaceans. Octopuses have salivary glands. Salivary glands inject venom into prey. Squid feed on fishes and shrimp. They kill them by biting across the back of the head.
The digestive tract of cephalopods is muscular. Peristalsis has replaced the ciliary action in moving food. Most digestion occurs in a stomach and a Cecum. Digestion is primarily extracellular. They have digestive glands supplying enzymes. An intestine ends at the anus, near the funnel. The exhalant water carries wastes out of the mantle cavity.
OTHER MAINTENANCE FUNCTIONS
Blood vascular system
Cephalopods have a closed circulatory system. Blood remains in vessels during its circulation in the body. Capillaries connect arteries and veins. The exchanges of gases, nutrients, and metabolic wastes take place through capillary wall. The heart of cephalopods consists of two auricles and one ventricle. The cephalopods have contractile arteries and branchial hearts. The branchial hearts arc present at the base of each gill. It helps in movement of blood through the gill. These modifications increase blood pressure and the rate of blood flow. Blood pressure is necessary for active cephalopods with high metabolic rates. Large quantity of water circulates over the gills at all times. Cephalopods show greater excretory efficiency due to close circulatory system. Th re is a close association of blood vessels with nephridia. It allows wastes to filter and sec tte directly from the blood into the excretory system.
The cephalopod have specialized nervous system. Cephalopods are predatorv animals. Therefore, they need efficient nervous system. Cephalopod brains are large. The brain is formed by a fusion of ganglia. Large areas of brain are used to control muscle contraction, sensory perception and memory and decision making.
1. Eye: The structure of eye of cephalopod is similar to the structure of vertebrate eyes. This is an example of convergent evolution. But the nerve cells leave the eye outside of the eyeball. Therefore, no blind spot exists in their eye. The cephalopods locus by moving the lens back and forth. Cephalopods can form images of different shapes. They differentiate between some colors. The nautiloid eye is less complex. It lacks a lens. Its interior is open to seawater. Thus it acts as a pinhole camera.
2. Statocysts: Cephalopod statocysts detect gravity and acceleration. Statocysts are in the form of cartilages next to the brain.
3. Osphradia: They are present only in Nautilus.
4. Tactile receptors and additional chemoreceptors are widely distributed over the body.
1. Chromatophores: Cephalopods have pigment cells called chromatophores. Tiny muscles attached to these pigment cells. These muscles contract and the chromatophores quickly expand. It changes the color of the animal. Color changes functions in alarm responses. Color changes may spread in waves over the body. It forms a large, flickering, patterns. The cephalopods blend with their background by changing color changes. Color changes are also involved with courtship displays. Some species combine chromatophores displays with bioluminescence.
2. Ink gland: All cephalopods possess all ink gland. It opens just behind the anus. Ink is a brown 9r black fluid. It contains melanin and other chemicals. This ink confuses a predator. Thus it allows the cephalopod to escape. For example, Sepiola reacts to danger by darkening itself with chromatophore before releasing ink. After ink discharge. Sepiola changes to a lighter color again. This change of colour helps in escape from the predator.
REPRODUCTION AND DEVELOPMENT
Cephalopods are dioecious. Their gonads are present in the dorsal portion of the visceral mass.
The male reproductive tract consists of testes and spermatophores. Spermatophores are used for covering the sperm in packets. The female reproductive tract produces large, yolky eggs. Glands secrete gel-like cases around eggs. These cases become hard in seawater.
One tentacle of male cephalopods is called hectocotylus. It is modified for spermatophore transfer. The hectocotylus has several rows of smaller suckers in Loligo and Sepia,. These suckers pick up spermatophores. Male and female tentacles intertwine during copulation and the male removes spermatophores from his mantle cavity.
The male inserts his hectocotylus into the mantle cavity of the female. It deposits a spermatophore near the opening to the oviduct. Spermatophores have an ejaculatory mechanism. It frees sperm from the capsule. Eggs leave the oviduct and fertilized. Fertilized eggs are deposited singly or in string like masses. They attach to some substrate. Octopuses clean developing egg with their arms.
Cephalopods develop in the egg membranes. The hatchings are similar to adults. Young are not cared after hatching.