Study Group Worksheet: chordates & vertebrates

Four defining characteristics of chordates:

o   Notochord

o   Dorsal, hollow nerve cord

o   Pharyngeal slits

o   Muscular post-anal tail

Present only during some life stages:

o   Any of the four (commonly the notochord or tail). For example, tunicate larvae have all four, but adults lose most.

Chordates retaining notochord for life:

o    Lancelets (Cephalochordata), hagfish (primitive craniates).

Dorsal nerve cord:

o   Located on the back (dorsal side), hollow, develops into the brain and spinal cord.

o   In contrast, many invertebrates (e.g., arthropods, annelids) have a solid ventral nerve cord.

Pharyngeal slits:

o   Openings between the pharynx and outside.

o   Functions: filter feeding (in tunicates/lancelets), gas exchange (fish gills), or other roles in vertebrates.

Post-anal tail:

o   Extends beyond the anus.

o   Used for locomotion in many aquatic species; in humans it’s reduced to the coccyx.

Craniate vs. Vertebrate:

o   Craniates: chordates with a head (skull, brain, sensory organs).

o   Vertebrates: craniates that also have a backbone of vertebrae.

Skeletal differences:

o   Vertebrates have a backbone made of vertebrae and a skull protecting the brain.

o   Skeleton is living tissue that grows (no molting).

Early chordates giving rise to vertebrates:

o   Cephalochordates & urochordates are closest relatives.

o   Vertebrates evolved heads, backbones, and more complex organs.

Chordates with notochord but no backbone:

o   Lancelets (Cephalochordata) and tunicates (Urochordata).

Urochordates (tunicates):

o   Larvae have all chordate traits but adults become sessile filter feeders, losing most chordate features.

Lifestyle differences:

o   Urochordata: sessile filter feeders (sea squirts).

o   Cephalochordata: active filter feeders, burrow in sand (lancelets).

o   Vertebrata: highly diverse, mobile, complex systems.

Evolutionary advantages of vertebrae:

o   Protect spinal cord.

o   Provide structural support.

o   Allow larger size and more active movement.

Active lifestyle adaptations:

o   Efficient circulatory system (closed, with heart).

o   Advanced nervous system (brain, sensory organs).

o   Endoskeleton supports growth and movement.

•

Evolutionary advantages of vertebrae:

o

Protect spinal cord.

o

Provide structural support.

o

Allow larger size and more active movement.

•

Active lifestyle adaptations:

o

Efficient circulatory system (closed, with heart).

o

Advanced nervous system (brain, sensory organs).

o

Endoskeleton supports growth and movement.