The classification of Appendicularia (Tunicata)

This is a study of the evolutionary relationships of a group of small planktonic marine animals, the Appendicularia or larvacea. They belong to the Tunicata, which includes the more familiar sea-squirts or Ascidia. The Appendicularia resemble the tadpole-like larvae of sea-squirts in having a `head' containing the brain, sense organs and gut, the latter having a mouth, two gill slits and an intestine. Thes rest of the body is made of a muscular tail, like that of a sea-squirt larva. The tail contains a notochord, a flexible structure like the embryonic form of the backbone in the vertebrates (animals with backbones: fish, amphibians, reptiles, birds and mammals, including humans). The Appendicularia thus belong to the chordates, which contains the vertebrates and their primitive relatives of the Appendicularians and sea-squirts are examples. Appendicularia secrete a mucous `house' which contains cellulose, which is otherwise only found in the `tunics' or outer `skins' of sea-squirts or other tunicates. The `house' absorbs water and expands to form a bubble-like structure surrounding the appendicularian, but which is many times larger than the appendicularian itself. Inside the house the tail waves from side to side, like that of an eel, for swimming and for making feeding currents. The currents enter a opening in the house, which traps food particles which are eaten by the animal. Excess water passes out of the gill slits. The house is discarded after a few hours or less as it become clogged with non-food particles, and a new one is secreted. Appendicularians have life-spans measured in days. A common appendicularian, Oikopleura dioica is currently becoming a widely-used research animal, as it has a very small number of genes and is easy to breed in captivity. This research should yield much information on genetics of chordates including the vertebrates.