Phyla Hemichordata and Chordata

Evolution of Deuterostomes

Phylum Hemichordata● Three body regions

-Proboscis-Collar-Trunk

● Deuterostomes

● Ciliated, pharyngeal gill slits

● Complete digestive system

● Dorsal and ventral nerve cord

Phylum Hemichordata

Acorn Worm

Phylum Hemichordata

● Acorn worms feed as they burrow

● Detritus and other material adheres to the proboscis and is moved to the mouth

● The posterior end is extended out of the burrow during defecation

● Coils of fecal material are called castings

Characteristics of Phylum Chordata● Bilaterally symmetrical

● Dorsal, hollow nerve cord

● Notochord – flexible, supportive rod parallel to dorsal nerve cord

● Gill slits or pharyngeal pouches

● Postanal tail

● Segmented muscles

● All deuterostomes

Phylum Chordata

Evolutionary Adaptations of Chordates

● From filter feeding ancestors to active predators. Major improvements occurred in the following areas:

● Mobility● Oxygen capture● Digestion● Circulation● Nervous system

Skeletal Changes

● Skeleton becomes stronger to work with bigger muscles

● Allows for more rapid movement

● Vertebrae develop around nerve cord and notochord for protection

Oxygen Capture

● Gill slits and muscular pharynx work to move more water over gills than occurred in lower animals

● Leads to more oxygen uptake for fueling a higher metabolism

Circulation

● Stronger heart to circulate blood faster

● Works to bring those higher oxygen loads to body tissues more quickly & efficiently

Digestion● Digest more kinds of food

more efficiently

● Muscularized gut grinds down food physically

● Liver and pancreas secrete substances that break down food chemically

● Leads to better overall efficiency

Nervous System

● More complex

● Better motor control allows for improved hunting and defense

● More finely tuned sensory receptors allows for clearer picture of environment

● Compartmentalization of brain allows for improved cognition and sensory input processing

Chordate Subphyla● Phylum Chordata

○ Subphylum Urochordata○ Subphylum Cephalochordata○ Subphylum Vertebrata

Subphylum Urochordata

● Sea squirts (Tunicates)

● Larvae are free-swimming, but adults are usually sessile

● Notochord found only in larvae. Dorsal nerve cord highly reduced in adult

● Adult body has a tough, though sometimes clear covering called a tunic

Subphylum Urochordata● Feeding and gas exchange

occurs via the same mechanism

● Water laden with oxygen and food enters via the incurrent siphon

● Oxygen & food is extracted by the pharyngeal gill slits

● Water and waste exits via the excurrent siphon

Larval Tunicate

Adult Tunicate

Subphylum Cephalochordata

● Small, tadpole-like animals collectively called lancelets

● Usually nearly translucent

● Notochord extends from the head to the tail in adults

● Live in shallow waters with clean, sandy substrates

Subphylum Cephalochordata

● Spend most of life with the body buried in the substrate & the mouth exposed to the water

● Cilia around the mouth called cirri pull water into the mouth and through the gill slits

● Food & oxygen are extracted in a similar manner to that seen in Urochordates

Subphylum Cephalochordata

Subphylum Vertebrata

● Characterized by vertebrae that form around the dorsal nerve cord & serve as the organism’s primary axial support

● All other characteristics of lesser chordates are conserved in vertebrates at some stage of their life cycles (embryonic)

Vertebrate Evolution

The End