Figure 13.2: Pikaia gracilens(subphylum cephalochordates), perhaps the oldest known an-
cestor of modern vertebrates, resembled a living chordate, known as a lancelet, and perhaps
swam much like an eel.Pikaiais thought to have had a very primitive, proto-notochord. Its
“tentacles” may be related to those in present-day hagfish, a jawless chordate. ( 29 )
What are Vertebrates?
Vertebrates, belonging to the subphylumVertebrata, are chordates with a backbone or
spinal column. Other characteristics are a braincase, orcranium, and an internal skeleton
(the latter feature is present in all vertebrates except for lampreys). All vertebrates are most
easily distinguished from all other chordates by having a defined head with pronounced
cephalization. Cephalizationis the concentration of nervous tissue towards one end of
the organism. Vertebrates have sensory organs, especially eyes, concentrated at the front
(anterior) end of the body. How do you think this type of body design is an advantage?
Typical vertebrate traits include:
- a backbone or spinal column
- braincase
- internal skeleton
- defined head with pronounced cephalization
- sensory organs, especially eyes
The vertebrate muscular system mostly consists of paired masses, as well as a central nervous
system, partly located inside the backbone, when a backbone is present. Extant (living)
vertebrates range in size from a carp species (Figure13.3), at as little as 7.9 mm (0.3 in),
to the blue whale, as large as 110 ft (Figure13.4).