Tag: phylum hemichordata

The roof of the oral hood in Amphioxus show the presence of Hatschek's pit which is responsible for the secretion of mucus. This mucus secretion helps to entrap food particles present in the water.

Balanoglossus is a hemichordate. The excretory organ in Balanoglossus is glomerulus. This is also known as the proboscis gland. This gland is present in front of the central sinus. The gland projects outward in the proboscis coelom. The gland is comprised of many tube like projections. The tubes are blind at one end. The tubes are formed by the peritoneum. They are present as the covering of buccal diverticulum, central sinus, and heart vesicle. This organism is considered as the link between vertebrates and invertebrates. 

Buccal diverticulum is a short and thick-walled projection which extend forwards through the proboscis stalk into the proboscis coelom. This is a hollow tube like projection. This is a tubular outgrowth from the mouth cavity forward into the proboscis.The wall is composed of a single layer of tall and vacuolated endodermal cell. Hemichordata is known as half chordate because of the buccal diverticulum. The structure resembles like a rudimentary notochord present in the dorsal or back side and supporting axis for primitive vertebrates. 

Early developmental stages of Balanoglossus possess a spacious coelom, lined with peritoneum. It is enterocoeles in origin, as the adult condition is attained, the coelomic epithelium gives rise to the connective tissue and muscle fibers. Hence, the coelomic cavity is obliterated. The coelom as a whole is divided into three parts. These include one cavity in proboscis, (protocoel), two in the collar, (mesocoel) and two in the trunk (metacoel).

A tornania is the planktonic larva of some species of Hemichordata. It is very similar in appearance to the bipinnaria larvae of starfishes, with convoluted bands of cilia running around the body. Hemichordata is a phylum of marine deuterostome animals.

The structure of 'tornaria larva' was first described by J. Muller in 1850. He suspected it to be the larva of starfish. Later, in 1869, Metchinkoff found that it was the larva of Balanoglossus clavigerus. The name tornaria was given to it because of its habitat of rotating in circles.

Protochordates are an informal category of animals (i.e., not a proper taxonomic group). This group has the animals that possess some but not all characteristics of chordates. They have pharyngeal gill slits and a dorsal nerve cord, however, they lack vertebral column and paired appendages.

Hemichordates share some but not all of the typical chordate characteristics and hence, are not true chordates.
There is a rudimentary structure in the collar region, the stomochord, that is similar to a notochord. There is a dorsal nerve cord, in addition to a smaller ventral nerve cord.

Branchiostoma is one of the few living genera of lancelets. They do not have a true head, but merely a mouth adjacent to the gill slits with the slightly enlarged anterior end of the dorsal nerve cord above and in front of them. In Brannchiostoma, the notochord is extended up to the head and is retained throughout its life.

The wheel organ is found in amphioxus, which is a specialized epithelium in the roof and sides of the adult lancelet oral cavity. It borders the oral epithelium proper. The wheel organ cells are tall and strongly ciliated and have dark, heterochromatin-rich nuclei. It draws food through the use of its cilia for digestion.