Protochordates
by Sarah Vorderbruggen, Colleen Rice, Breanna Leitzke
- What is a protochordate? Why are they significant to a study of vertebrate evolution?
- Below we describe three protochordates, their basic anatomy and biology.
Protochordates are a group of soft bodied invertebrate deuterostomes with a small mouth and large pharynx with some or all of the five diagnostic characteristics of a chordate. They are significant to the study of vertebrate evolution because they can give an idea of the anatomy and biology of the earliest vertebrates.
Protochordates include: acorn worms, amphioxus, and tunicates.
Acorn Worm
https://www.youtube.com/watch?v=f0J3qJCvUUY
Phylum: Hemichordata
Acorn worms are “worm-like” animals with a 3 part body plan, consisting of a proboscis, collar, and trunk. They have a stomochord, a dorsal nerve cord, and pharyngeal slits on the trunk.
When feeding, the acorn worm will stick its head out of the burrow and filter feed by trapping food in the mucus on the proboscis and directing the food into the mouth by cilia. Other species will ingest food suspended in their burrows.
Food then goes into the pharynx and the water exits through the pharyngeal slits. The food is digested in a simple intestine and then expelled.
Amphioxus
Phylum: Chordata, Subphylum: Cephalochordata
Follow the basic “fish” body plan, complete with: all five chordate characteristics in both the larval and adult stage, a wheel organ with buccal cirri located in the oral hood, and segmental muscles.
They contain a closed, tubular circulatory system but lack a heart or muscular pump. They also contain a partial excretory system.
Amphioxus are filter feeders. Food is processed through the organs of the oral hood and into the pharynx. After being processed through the intestine, waste leaves the atriopore.
Tunicates (Sea Squirt)
Phylum: Chordata, Subphylum: Urochordata
The tunicate is a circular shape with incurrent and excurrent siphons. The pharynx with longitudinal bars is directly connected to the incurrent siphon, and the excurrent siphon functions as the atriopore. The tunicate also possesses a gonad, stomach, intestine, heart, tunic and mantle.
The tunicate is a filter feeder. Food enters the incurrent siphon and processes through the pharynx to be digested by the stomach. The food then passes through the intestine and out the excurrent siphon as waste.
The larval stage of the tunicate is called the ascidian tadpole and possesses pharyngeal slits, an endostyle, a notochord, a nerve cord, and a post anal tail. The mature stage of the tunicate possesses pharyngeal slits and an endostyle.
