Danny Meyer, age 12, of St. Matthews, Kentucky, for his question:
What is a decapod?
Scientific terms are fun to decode and the word decapod is one of the best. The prefix dec means ten, as in decimal. However, in decapod the C is pronounced like a K. The. A that follows the deca indicates a state of being or having, as in alive. The last syllable is the scientific term for a foot.
Biologists have classified the world of plants and animals into group according to their characteristics and have given them meaningful scientific names, usually coined , from Latin or Greek. Each group, large or small, and each individual species has an identification tag of its own. Or so goes the rule. Exceptions, they say, prove the rule and this old saying applies to the term decapod. Two entirely different types of animals belong to the name decapod.
The term decapod means ten footed and it is reserved for animals that have ten feet, arms or legs, It is a good word, neat and meaningful. But for some reason, perhaps an oversight or a failure to cross check, Decapoda was used to name two entirely different animal orders. The two Decapoda orders even belong in different phyla, showing that one group of decapods is vastly different from the other.
One Decapoda belongs in the Phylum mollusca, along with the snails and clams.. This Decapoda is an order or suborder in the Class ~Cephalopoda, which includes the octopus. The decapods of this order have ten leggy tentacles, fitted with round, gripping suction cups. Some have internal bony plants and a few wear graceful external shells to protect their soft, gristly bodies. We rarely see them because most of them live deep in tropical oceans. Better known members of this order include the jet propelled squids of coastal water, the inky cuttlefish and the giant squid.
The second, and very different, group of decapods belong in the Phylum Arthropods, along with the spiders and centipedes. Their Decapoda Order is one of several in the Class Crustacea. They are crustaceans with special features of their own. If you guessed that these decapods are ten legged critters with jointed shells, you would be correct. The head and thorax are shielded in a crusty carapace with gill openings along its sides. Their ten jointed legs are encased in crusty hose and boots made for walking. In some cases, the two front legs bear one or a pair of mighty pincers mean but useful. By now you have guessed that this group of decapods includes lobsters and shrimps, crabs and crayfish.
The two decapod groups are not all related to each other. But each of them has ten appendages of some kind. Almost all of them are water dwellers fitted with gills and most of them may meet their decapod relatives and nonrelatives in the ocean, although a crayfish may prefer fresh water. Both groups of decapods have family trees dating back 180 million years. All the decapod ancestors arrived on earth with the early dinosaurs during the Triassic Period of the earth's history. But even the two groups of remote ancestors were not related to each other.