Phylum Chordata The Lower Chordates

The Phylum Chordata Includes: Subphylum Cephalochordata (=lancelets) Subphylum Urochordata (= tunicates) 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

Sea squirt, Halocynthia

Subphylum Urochordata continued 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

Lancelet, Branchiostoma

Subphylum Cephalochordata 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.

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