The Cephalopods are the familiar squids, cuttlefish and octopii, and the bizarre Nautilus. These animals are the pinnacle of the mollusc evolutionary line. They are possibly the most intelligent of the invertebrates, and possess excellent eyes, nervous, and propulsion systems. The foot is modified into tentacles (8 in octopi, 10 in squids and cuttlefish, and 80+ in nautilus), and the mantle is modified into a sort of "jet propulsion" system that helps these active predators move around. 

Cephalopods are very inquisitive and bright, and must be placed into an aquarium that is designed just for them. All require excellent filtration, as they must eat frequently and are very intolerant of poor water quality. A strong mechanical filter is necessary to insure good water quality as these animals are messy eaters. To insure good water chemistry, change about 20 percent of the tank's water every 2 weeks. They usually enjoy live food such as small fish or crustaceans, but can be trained to eat dead food from your hand, (but make sure that your particular species does not bite humans). Feed them 2-3 times a week, or they may start to eat their tank mates. As a rule, do not put cephalopods with any other actively moving critters, as they may decide to eat them. The 650+ species will lay eggs that give rise to fully formed juveniles. Most species of octopus are good parents, while squids and cuttlefish leave their eggs unattended, and almost all cephalopods shortly after breeding. 

The radula of these animals is modified into a sharp parrot like beak, and some species (including the deadly Blue Ring Octopus) use venom to stun or kill their prey. Most species will fight with each other, so only one per tank is the rule. The specific tank design for octopus and cuttlefish are slightly different. Here are some more hints to help you keep these organisms happy. A tank for octopus should have plenty of places to hide. A small cave will do fine. Cuttlefish like hiding places as well, but may also want to bury themselves in the sand. The octopus is the supreme escape artist (although less so, the same is true for cuttlefish) so insure that tank lid is secure. Also insure that any filter intakes have good sized diffusers to slow the rate of suction. These are high strung animals that do not enjoy disturbances, so do not tap on the glass or harass them in any way. They also need a period of darkness, and many species are actually nocturnal (active at night). Put your aquarium on a 14 hour on 10 hour off light cycle. Acclimate the new animal to its tank very slowly and gently. Turning off the lights prior to acclimation will help to soothe their nerves. If your animal looks sluggish or ill, check both water quality and the temperature; as these are the two most critical factors influencing their health. While these animals seem demanding in that most require their own tank, I think you will agree that these interesting animals are well worth their own home.