Invertebrate chordates, or prochordates, are those in which the notochord is not replace by a vertebral column during development. The two subphylum of prochordate are the lancelets (subphylum Cephalochordata) and the tunicates (subphylum Urochordata).
The lancelets are the simplest of organisms displaying the four chordate characteristic. The notochord, dorsal tubular nevre chord, gill slits, and tail are all seen in the adult form. A lancelet is very mobile, and powers its way through the water with muscles that use the notochord as a point of attachment. The tenticles around the mouth are for searching through mud for microscopic food particles that will filter out of the constant waterstream that is moving through the animal.
The adult tunicate looks more like an invertebrate that a chordate, since the only retained trait from the larval stage are the gill slits, but the larva posseses all of the characteristics. The tadpole-like larvae is bylaterally simetrical and free swimming; therefore, it is the dispersal stage of the tunicate life cycle. The larvae will eventually settle on the sea floor and metemorphisize to the sessile adult tunicate. The tunicate is also a filter feeder, and draws water throught its incurrent siphon, and past the gill slits to filter out oxygen and food. The larvaceans (class Appendicularia) are a group of tunicates that remain in the larval form and do not become the sessile adult. It is theorised that this tunicate would be the like to the freeswimming vertebrate fishes.
1) Why is a tunicate considered a prochordate?