General Characteristics
Foot has been modified into a circle of tentacles/arms used for prey capture, attachment, locomotion, and copulation. Foot is incorporated into a funnel associated with the mantle cavity and is used for jet-like locomotion. Cephalopod head is in line with the visceral mass. Mantle acts as a pump tpo bring large quantities of water into the mantle cavity.
Shell
Ancestors had a conical shell. Nautilus is the only living cephalopod known to possess an external shell. As nautilus grows, it moves forward secreting new shell around itself and leaving an empty septum behind. When formed these chambers are fluid filled. In other cephalopods, the shell is reduced or absent. Cuttlefish have an internal shell and is laid down in thin layers, leaving small gas-filled spaces that increase buoyancy.
Locomotion
Depend on quick jet propulsion system. Mantle contains radial and circular muscles. When circular muscles contract they decrease the volume of the mantle cavit and close collarlike valves to prevent water from moving out of the mantle cavity. Water is then forced out of a narrow funnel. Radial mantle muscles bring water into the mantle cavity by increasing cavity's volume. Posterior fins act as stabilizers. Escape usually by crawling under substrate via tentacles but can also use jet propulsion. Rigid external shell would preclude the jet propulsion method of locomotion
Feeding and Digestion
Most prey are located with sight and captured by tentacles with adhesive cups. Squid have cups reinforced with tough protein and sometimes possess small hooks. They have jaws and radula. Jaws are powerful beaklike structures for tearing food, and radula rasps food forcing it into the mouth cavity. Cuttlefish and natiluses feed on small invertebrates on the ocean floor. Octopuses are nocturnal and have salivary glands that inject venom into prey. Squid kill by biting across the back of the head. Digestive tract of cephalopods is muscular, and peristalsis (coordinated muscular waves) replaces ciliary action in moving food. Most digestion occurs in a stomach and a large cecum. Digestion is primarily extracellular, with large digestive glands supplying enzymes. An intestine ends at the anus, near the funnel, and exhalant water carries wastes out of the mantle cavity.
Maintenance and Functions
Closed circulatory system confines blood to vessels. Capillary beds connect arteries and veins, and exchanges of gases, nutrients, and metabolic wastes occur across capillary walls. Heart consists of two auricles and one ventricle, cephalopods have contractile arteries and branchial hearts loacted at the base of each gill. These modifications increase blood pressure and blood flow rate enabling high metabolic rates for highly active animals. Closed circulatory increases excretory efficiency. Close association of blood vessels with nephridia allows wastes to filter and secrete directly from the blood into the excretory system. Its nervous system includes a large brain formed by a fusion of ganglia
Learning
Largest brain to body weight ratio out of all invertebrate, fish, or amphibian; octopus brains have vertical and superior frontal lobes are learning and MEMORY centers for visual stimuli. They can learn through observation and conditioning. Reasons for higher sentience may be to avoid predators while being an active predator itself. Capable of multitasking, example, flashing cryptic and courting patterns simultaneously.
Reproduction and Development
Dioecious with gonads in the dorsal portion of the visceral mass. Males have testes and spermatophores packets that encase sperm. Females produce large, yolky eggs and is modified with glands that secrete gel-like cases around eggs. These cases frequently hard on exposure to seawater. Males have a tentacle called hectocotylus transfers spermatophore. The hectocotylus uses several rows of small suckers to do so. Males and females copulate by intertwining their tentacles and males removes their spermatophores from their mantle cavity. THe male inserts his hectocotylus into the mantle cavity of the female and deposits a spermatophore near the opening to the oviduct. Spermatophores have an ejaculation mechanism that frees the sperm to reach the oviduct and fertilizes the eggs which are deposited singly or in stringlike masses which attach to a substrate like the ceiling of an octopus den. Octopus can clean the eggs with their arms and squirts of water. Cephalopods develop in the confines of the egg membranes, and the hatchlings are miniature adults. The young are independent after hatching.