HEMICHORDATA-PTEROBRANCHIA

Solitary or colonial,sessile and tubicolous animals living inside secreated chitinous tubes
Body short,compact,with stalk for attachement..
Proboscis shield-like.
Collar bearing ciliated arms.
Alimentary canal u-shaped
Anus dorsal lying near mouth
Ciliary feeding..
Gill-slits one pair or absent.
Sexes separate or united.
Gonads 1 or 1.
Asexual reproduction by budding in some.

HEMICHORDATA-CLASSIFICATION

Hemichordata includes about 80 species include Enteropneusta and Pterobranchia . 

Enteropneusta

Solitary ,free-swimming or burrowing animals commonly called the "acorn" or "tongug worms".

Body elongated,vermiform.

Proboscis cylindrical and tapering.

Collar without ciliated arms(lophophore).

Alimentary canal straight.

Mouth and anus at opposite ends.

Filter feeding.

Several pairs of u-shaped gill-slita.

Sexes separate.

Gonads numerous,sac-like.

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HEMICHORDATA-CHARACTERS

It include marine,solitary or colonial and mostly tubicolous.
Body is soft, fragile,vermiform,unsegmented,bilaterally symmetrical and triploblastic.
Body is divided into three regions proboscis,collar,and trunk.
Body wall of a single-layered epidermis with mucuous glands.
Coelom is enterocoelous, usually divided into protocoel,mesocoel and metacoel.
Digestive tube complete,straight or u-shaped.
Ciliary filter feeders.
Circulatory systems simple and open.
Excretion by a single proboscis gland.
Nervous system primitive.Dorsal collar nerve cord hollow.
Reproduction mainly sexual.
Fertilization external in sea water.Leads to tornaria larva.

Balanoglossus-AFFINITES WITH CHORDATA

In 1885 WILLIAM BATESON proposed closer affinites with chordata and non-chordata.The resemblances  shows on the presence of a notochord,nerve chord, and pharyngeal gill-slits(pharyngotremy).In cephalochordata and urochordatd the structure and function of pharynx and branchial apparatus are similar.In Branchiostoma the origin of coelom is enterocoelic.soHemichordata had been considered as subphylum of the chordata.

Balanoglossus-AFFINITES &SYSTEMATIC POSITION OF BALANOGLOSSUS

The group ENTEROPNEUSTA, to which BALANOGLOSSUS belongs,was established by GEGENBAUR in 1870.BATESON(1885) proposed the name Hemichordata in place of enteropneusta. Due to their peculiar anatomical organisation and embryology, the hemichordata (balanoglossus) have been closer to the chordata and non-chotdata.

Balanoglossus-METAMORPHOSIS

The larva swims freely,leads a plantonic life feeding on minute organisms, and metamorphoses into an adult worm. During metamorphosis, the size is reduced probably due to loss of water. Transparency, ciliary bands, sensory cilia and eye spots are lost. Body becomes differentiated into roboscis,collar and trunk by the aearance of two constrictions, and the trunk region is elongetedd. The hydroore ersists as proboscis pore, and the buccal diverticulum and gill-slits appear as outgrowths of the alimentary canal. The animal now sinks to the bottom to lead a benthonic life as an adult.

Balanoglossus-TORNARIA LARVA

Tornaria larva was first described by J.MULLER in 1850 the Balanoglossus clavigerus.size 3 mm in size.It is glossy in appearance with an oval body having ventral mouth near the equatorial plane of the body, a posterior terminal anus and gut differentiated into an oesophagus, stomach and intestine. The cilia form two bands on the body surface. The anterior ciliaryn band or cirumoral band takes up a winding course over the preoral surface and forms a postoral loop;its cilia are short and serve to collect food. The posterior ciliated band or telotroch occurs as a ring in front of the anus;its cilia are long and serve as locomotor organs. At the anterior end is an apical plate of thickened epidermal cells,which bears a pair of eye spots or ocelli and a tuft of sensory cilia called apical tuft or ciliary organ. The protocoel(proboscis coelom) in the form of a thin-walled sac,is present and opens to the exterior through a hydropore(proboscis proe).To the right of the hydropore lies a pulsating heart vesicle.The collar and trunk coeloms appear in the older larva.