BRANCHIOSTOma-INTESTINE

The gut is long as the pharynx .It is suspended from the dorsal bodywall by a dorsal mesentery into the atrial cavity. It is covered by a thin layer of smooth muscles and its internal epithelial lining has several ciliated tracts. The intestine can differentiated into three parts midgut. The large blind pouch or diverticulum arises from the juctions of oesophagus and midgut. The lining of midgut on the right lateral side bears a crescentic lateral ciliary tract. Its cilia beat downwards directing food into the midgut diverticulum. Midgut is followed by a densely ciliated ilio-colic ring .The hind gut is narrower.The terminal part or rectum is heavily ciliated and opens to outside by anus.

BRANCHIOSTOMA-OESOPHAGUS

The pharynx opens into a short narrow and tubular oesophagus having ciliated internal lining and leading into the gut.

BRANCHIOSTOMA-PERIPHARYNGEAL BANDS

These are a pair of narrow ciliated tracts from the anterior end of endostyle. They are upward and one along the lateral wall of pharynx to unite dorsally with the anterior end of epipharyngeal groove. These bands are a small ,antero-dorsal prebranchial region of pharynx devoid of gill-slits.

BRANCHIOSTOMA--EPIPHARYNGEAL GROOVE

The hyperpharyngeal is aprominent ciliated groove present mid-dorsally along the roofof pharynx. It runs upto the oesophageal opening.

BRANCHIOSTOMA-ENDOSTYLE

The hypobranchial tract extends mid ventrally the entire floor of pharynx. It forms a shallow groove with four longitudinal tracts of mucus-secreating gland cells with five tracts of ciliated cells. The cilia of the median tract are the longest. The endostyle is supported below by two gelatinous skeletal plates under which is present the sub-endostylar coelomic canal contaning the ventral aorta.

BRANCHIOSTOMA-PHARYNGEAL GILL BARS

The pharyngeal gill bars are of two type primary and secondary- The skeletal rod pf a primary gill bar is called primary gill rod . Its lower end is forked and it encloses a blood vessel and a coelomic canal through out its length. The secondary gill bar develops a small tongue bar which grows downwards from the dorsalwall of a primary gill slits. The skeletal rod  of a secondary gill bar is called secondary gill rod.The primary gill bars are interconnected by a system of transdverse  bars or synapticula which impart a basket like appearance to the pharynx similar to the branchial sac of tunicates. A synapticulum also contains a blood vessel and a skeletal rod.

BRANCHIOSTOMA-PHARYNGEAL WALL AND GILL SLITS

The lateral walls of pharynx are perforated by 150-200 pairs of closely and obliquely set narrow and vertical opening called branchial apertures gill-slits or gill-clefts. The gill-slits bear no gill and their number increages with the age of the animal on the posterior .The portion of pharyngeal wall between two gill slits is called a branchial lamella or gill bar. Gill bar consists of a ciliated epithelial covering both ectodermal, enclosing a mesodermal core containing fibrous connective skeletal rod.