+

Phylum Chordata

The Lower Chordates

+The Phylum Chordata Includes: 1. Subphylum Cephalochordata (=lancelets)

2. Subphylum Urochordata (= tunicates)

3. Subphylum vertebrata:• Class Agnatha - jawless fish • Class Chondrichthyes - cartilaginous

fish• Class Osteichthyes - bony fish• Class Amphibia• Class Reptilia• Class Aves – birds• Class Mammalia

+Introduction to Chordata Objectives

State the 4 characteristics of all chordates

State the 4 Subphyla and an example animal

+Lower Chordates

These animals have the characteristics of chordates at some point in their lifecycle

For this reason, they are referred to as the “ Lower Chordates”

+4 main characteristics

1. Notochord

2. Hollow dorsal nerve chord

3. Pharyngeal gill slits

4. Muscular, post anal tail

NOTE: NOT all characteristics are seen in all adults – but are present at some point in life cycle

+Notochord:A long flexible supporting rod that runs through at least part of the body – usually along the dorsal surface just beneath the nerve cord.

Muscles can attach to notochord

Present usually early in development to be replaced by a backbone in vertebrates.

+Hollow Dorsal Nerve Cord:

Runs along the dorsal surface just above the notochord (in most invertebrates nerve cords run along the ventral surface).

Nerves leave this cord at regular intervals to connect with organs, muscles, and sense organs.

The front end of the cord, in most chordates, develops into a brain.

+Pharyngeal slits:

Paired structures in the throat region of the body.

In aquatic chordates – lancets and fishes – these are gill slits.

Terrestrial chordates use lungs for respiration, but have pharyngeal slits for a brief period of time during the development of the embryo.

+Post-anal tail:

A tail extending beyond the anus, it is found in most chordates and contains skeletal elements and muscles.

Subphylum Urochordata

The Tunicates

All are marine

Only the tadpole-shaped larvae of tunicates have a notochord and a dorsal nerve cord

The larvae are very small but highly motile tadpoles.

It uses this stage for species distribution

Having a highly motile, feeding larva is a real advantage but it only lives a short while (1 - 2 days) and doesn’t reproduce

Subphylum Urochordata continued

+ 14

Sea squirt, Halocynthia

Subphylum Urochordatacontinued

After larvae undergo metamorphosis NO notochord and NO dorsal nerve chord become sessile, attach to the floor, and live

in colonies. These animals filter feed and breathe at

the same time through a pharyngeal basket pierced by gill slits

Adult Tunicate

+ Subphylum Cephalochordata

often called the “classical chordate” as it is the only animal to have ALL chordate characteristics as an adult

Animals also have segmentation

believed to share a common ancestery with the vertebrates

+ 18

Lancelet, Branchiostoma

+

The Lancelets:

Live in the sandy body of shallow tropical oceans. They are fishlike with a definite head.

Feeding – pass water through their pharynx where food gets stuck in mucus. This mucus is then swallowed into the digestive tract that passes straight to the anus near the tail. Lancelets have no jaw.

Respiration – they have a mouth that opens to a long pharyngeal region with up to 100 pairs of gill slits.

Subphylum Cephalochordata

+Lancelets Con’t:

Vascular system – Simple primitive heart that pumps blood through vessels in a closed circulatory system.

Show evidence of segmentation as their muscles are organized into V – shaped units that are on either side of the body. Each unit receives a branch from the main nerve cord. This segmented nerve and muscle organization is found in all living vertebrates.

Movement – they have no appendages and move by bending their bodies back and forth.

+Subphylum Cephalochordata

Amphioxus sp. A Cephalochordate