istics. About 58,000 species have been described and contain many familiar groups of large
land animals.
Chordates
Chordates (phylum Chordata), including the vertebrates and several closely related in-
vertebrates, are united by having, at some time in their life cycle, anotochord,a hollow
dorsal nerve cord); pharyngeal slits (vertical slits in the pharynx wall, which help to filter
out food particles); anendostyle(ciliated groove or grooves located in the pharynx), and a
post-anal tail. The phylum is broken down into three subphyla: Urochordata (represented
by tunicates), Cephalochordata (represented by lancelets) and Vertebrata (the vertebrates).
Urochordateshave a notochord and nerve cord only during the larval stage andcephalo-
chordateshave a notochord and nerve cord but no vertebrae (bones in the backbone). In
all vertebrates, except for hagfish, the notochord is generally reduced and the dorsal hollow
nerve cord is surrounded with cartilaginous (made of cartilage, not bone) or bony vertebrae.
The urochordates consist of 3,000 species of tunicates (sessile (permanently attached) marine
animals, with saclike bodies having thick membranes and siphons for water movement) and
the cephalochordates consist of 30 species of lancelets (burrowing marine animals). The
vertebrates encompass 57,739 species, including jawless and jawed vertebrates.
The origin of chordates is currently unknown. The first clearly identifiable chordates appear
in the Cambrian Period (about 542 - 488 million years ago) as lancelet-like specimens.
Figure 13.1: Tunicate colonies ofBotrylloides violaceus(subphylum urochordata), showing
oral tentacles at openings of oral siphons, which take in food and water, and expel waste
and water. ( 14 )