Have you ever heard about ancestors of chordates

The ancestors of chordate thought to be hemichordates, pogonophorans.  As you know chordates are animals who have nerve chord. The important facts is all vertebrate are chordates but all chordates are not vertebrates. Vertebrates have a backbone. So these hemichordates are half chordates as they have a stomochord similar in composition to notochord.

 

They are usually known as acorn warms. So they are warm shaped, sedentary, marine animals who live in shallow water or in secreted tubes while constructing helical borrows on silty-sand as colonies. They are deuterostome animals hence they have a bilaterally symmetry with indeterminate cleavage and a mouth that does not arise from the blastopore. 

 

They date back to the Lower or Middle Cambrian and include an important class of fossils called graptolites, most of which became extinct in the Carboniferous. Living hemichordates are a phylum with two living classes such as, the Enteropneusts and the Pterobranchs. The Acorn worms are Enteropneusta Hemichordates class of invertebrates, closely related to the chordates. There are about 70 species of acorn worm in the world. Some of these worms may grow to be very long like length of 2.5 meters. Pterobranchia is a clade of small, worm-shaped animals. They belong to the Hemichordata, and live in secreted tubes on the ocean floor

 

The bodies of Hemichordates are divided into three parts such as short, blunt, anterior proboscis, a short collar and a long posterior trunk. They have soft, elongated and warm like bodies. Also they bear a row of gill pores for respiration on each side of the anterior region of the trunk. They have open circulatory systems and a complete digestive tract. They have suspension feeding pattern, and food is mostly transported through it by using the cilia that cover its inside surface. There is another interesting fact, an iodine like order is characteristic of acorn warms.  

 

 

 

 

 

 

by Nuwandara Mudalige

Photo source : Internet