Sea Squirts

Tunicates, also known as urochordates, are members of the subphylum Tunicata, previously known as Urochordata, a group of underwater saclike filter feeders with incurrent and excurrent siphons that is classified within the phylum Chordata. While most tunicates live on the ocean floor, others such as salps, doliolids and pyrosomes live above in the pelagic zone as adults. Various species are commonly known as sea squirts[1] or sea pork.[2]

Most tunicates feed by filtering sea water through pharyngeal slits, but some are sub-marine predators such as the Megalodicopia hians. Like other chordates, tunicates have a notochord during their early development but lack myomeric segmentation throughout the body and tail as adults. Tunicates lack the kidney-like metanephridial organs, and the original coelom body-cavity develops into a pericardial cavity and gonads. Except for the pharynx, heart and gonads, the organs are enclosed in a membrane called an epicardium, which is surrounded by the jelly-like mesenchyme. Tunicates begin life in a mobile larval stage that resembles a tadpole, later developing into a barrel-like and usually sedentary adult form.

Tunicates apparently evolved in the early Cambrian period, beginning some 540 million years ago. Despite their simple appearance, tunicates are closely related to vertebrates, which include fish and all land animals with bones.