PHYLUM HEMICHORDATA (hemi, half + chorda, cord)
The phylum Hemichordata includes the acorn worms (class Enteropneusta) and the pterobranchs (class Pterobranchia. Members of both classes live in or on marine sediments. Characteristics of the phylum Hemichordata are:
1. They are marine deutrostome animals.
2. Their body is divided into three regions: proboscis, collar, and trunk.
3. Coelom is also divided into three cavities
4. They have ciliated pharyngeal slits
5. They have open circulatory system
6. They have complete digestive tract
7. In some cases, dorsal tubular nerve cord is present.
CLASS ENTEROPNEUSTA (acorn worms) (entero, intestine + pneustikos, for breathing)
Habit and habitat: Members of the class Enteropneusta are marine worms. They range in size between 10 and 40 cm. Some can be 2 m. Zoologists have described about 70 species. Most of them occupy U-shaped burrows in sandy and muddy substrata. These substrata are present between the limits of high and low tides.
Body parts: Their body is divided into three parts:
1. Proboscis: The common name of the enteropneusts is acorn worms. This name is derived from the appearance of the proboscis (-kr). Proboscis is a short, conical projection at the anterior end of worm.
2. Collar: A ring like collar is present posterior to the proboscis.
3. Trunk: An elongate trunk is the third division of the body.
A ciliated epidermis and gland cells cover acorn worm. The mouth is located ventrally between the proboscis and the collar. A variable number of pharyngeal slits are present late ally on the trunk. Their number is from a few to several hundred. Pharyngeal slits are openings between pharynx and the outside of the body.
1. Ingestion: Cilia and mucus help acorn worms in feeding. Detritus and other particles attach to the mucus-covered proboscis. Tracts of cilia transport posteriorly and ventrally. Ciliary tracts converge near the mouth. It forms a mucoid string. This string en rs the mouth. Acorn worms pull the proboscis against the collar to reject some substances. Ciliary tracts of the collar and trunk transport rejected material and discard it posteriorly.
2. Digestion: The digestive tact of enteropneusts is a simple tube. Hepatic sacs release enzymes. Hepatic sac is diverticula of the gut. These enzymes digest food. The worm extends its posterior end out of the burrow during defecation. Fecal material is called castings. These castings are present at low tide on the substrate at burrow openings.
The nervous system of enteropneusts is ectodermal in origin. It lies at the base of the ciliated epidermis. It consists of dorsal and ventral nerve tracts and a nerve plexus. In some species, the dorsal nerve is tubular. It contains giant nerve fibers. These fibers rapidly transmit impulses. Thereare no major ganglia. Sensory receptors are unspecialized. They are widely distributed over the body.
Respiration and Excretion
Acorn worms are small. Therefore, respiratory gases and metabolic wastes are exchanged by diffusion though the body wall. Their waste product is ammonia. Respiratory gases are exchanged at pharyngeal slits. Cilia are associated with pharyngeal slits. These cilia circulate water into the mouth and out of the body through the pharyngeal slits. Water passes through these pharyngeal slits. Gases are exchanged by diffusion between water and blood sinuses. These sinuses surround the pharynx.
Blood vascular system
Their circulatory system consists of one dorsal and one ventral contractile vessel. Blood moves anteriorly in the dorsal vessel. But it moves posteriorly in the ventral vessel. Branches from these vessels enter into open sinuses. All blood flowing anteriorly passes into a series of blood sinuses, called glomerulus. Glomerulus is present at the base of the proboscis. Excretory wastes are also filtered through glomeruli. These wastes are filtered into the coelom of the proboscis. These are released to the outside through one or two pores in the wall of the proboscis. The blood of acorn worms is colorless. It lacks cellular elements. It distributes nutrients and wastes.
Reproduction and Development
Enteropneusts are dioecious. Two rows of gonads lie in the body wall in the anterior region of the trunk. Each gonad opens separately to the outside. Fertilization is external. Spawning of one worm induces others to spawn. This mouth behaviour is controlled by some spawning pheromones. They produce ciliated larvae called tornaria. It swims in the plankton for several days to a few weeks. The larvae settle to the substrate. It gradually transform into the adult form.
CLASS PTEROBRANCHIA (pteron, wing or feather + branchia, gills)
Habitat: Pterobranchia is a small class of hemichordata. They are found in deep, ocean waters of the Southern Hemisphere. A few live in Europe coastal Waters and in shallow waters near Bermuda. Zoologists have described approximately 20 species of pterobranchs.
Size and body parts: Pterobranchs are small animals. Their size ranges from 0.1 to 5 mm. Most live in secreted tubes in asexually produced colonies. The pterobranchs body is divided into three regions.
1. Proboscis: The proboscis is expanded and shield like. It secretes the tubes and helps in movement in the tube.
2. Collar: The collar possesses two to nine arms. These arms have numerous ciliated tentacles.
3. Trunk: The trunk is U-shaped.
Pterobranchs use water currents. This current is generated by cilia on their arm and tentacles. Cilia trap and transport food particles toward the mouth. One genus has a single pharyngeal slits. But gases and wastes are exchanged by diffusion. Therefore,respiratory and excretory structures are unnecessary.
Reproduction and Development
Asexual budding is common in pterobranchs. They form colony by budding. Pterobranchs also possess one or two gonads in the anterior trunk. Most species arcs. External fertilization takes place. Planula like larva develops in them. This es for some time in the tube of the female. This nonfeeding larva then leaves the tube. It settles to the substrate. It forms a cocoon and metamorphoses into an adult.