While the chordata share many developmental features, their life histories and ecology vary greatly. For instance, adult urochordates are typically sessile, while many vertebrates are quite active. For this reason, the three subphyla, Urochordata, Cephalochordata, and Vertebrata, will be treated separately. The life history and ecology of the vertebrates will not be treated here, but under their own section.
Urochordata - Commonly known as "sea squirts" or tunicates, these are a marine group. In most groups, the young tunicate, called a larva, resembles a tadpole and is able to swim freely by means of a tail. As it matures, the larva settles and metamorphoses into a sedentary adult -- losing its tail, its ability to move, and many of its chordate features. The nervous system disintegrates, and the adult form becomes essentially a sack with two siphons through which water enters and exits. This water is filtered through a sac inside the bag-shaped body to procure food for the organism. Reproduction may be asexual through budding, or sexual. In the latter case, the tunicate is hermaphroditic, possessing both sets of sex organs.
Cephalochardata - The lancelets are found in coastal waters of tropical and temperate regions. Amphioxus (Branchiostoma) is the more common member of the group, and has received the common name "lancelet" because of its short tapered body. While capable of swimming, the lancelet has no complex sense organs, and so it burrows into the sand of shallow waters where the currents keep the environment from becoming stagnant. There it rests with only the front end exposed to the water, using a row of tentacles to bring food into its mouth. The sexes are separate in this group; the sex organs occurring in pairs along the body and producing gametes which are released into the water at maturity.
