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.

BRANCHIOSTOMA-PHARYNX

Pharynx is a very large, spacious and compressed chamber forming the largest part of alimentary canal. It occupies nearly one-half anterior part of body and suspended in the atrial or peribranchial cavity which surrounds it on all sides except the dorsal.

BRANCHIOSTOMA-VELUM AND ENTEROSTOME

The vestibule is closed by a circular ring-like vertical membrane the velum. It is perforated by a central circular aperture the enterostome. The enterostome is erroneously called mouth .The true mouth opens intostomodaeum lined with ectoderm. The velum is provided with a sphincter to open or close the enterostome. The posterior border of velum is prouced into 10 to 12 slender , ciliated and sensory velar tentacles.They also serve to strain the water current entering through the enterostome.

BRANCHIOSTOMA-WHEEL ORGANS

The epithelial lining of oral hood forms 6 to 8 pairs of finger -like folds each formed by a ciliated groove bounded by a ciliated  ridge. Collectivelty these are called as wheel organs or muller's organs. In rotifers the cilia of wheel organ set up a vortex or whirling water current to sweep food organisms into mouth. The mid-dorsal groove is the largest which ends in a small depression on the roof of buccal cavity. These are called Hatschek's groove and Hatschek's pit. Both are ciliated ,glandular and secreate mucus while the pit is considered a sensory organs .

BRANCHIOSTOMA-VESTIBULE

The oral hood encloses a large funnel-shaped cavity or vestibule into which opens the mouth. The cavity is lined with ectoderm, and is regarded as stomodaeum and its external opening the true mouth.

BRANCHIOSTOMA-BUCCAL CIRRI

In branchiostoma 10 to 15 pairs of stiff slender and ciliated oral or buccal cirri are present which bears sensory papillae. Their number increase with age. The buccal cirri and the edge of oral hood supported by stiff, gelantinous skeletal rods. The buccal cirri form a sieve to filter entry of large particles with food current.

BRANCHIOSTOMA-ORAL HOOD AND BUCCAL CAVITY

The oral hood is formed by the dorsal and lateral projections of the anterior end of trunk.

BRANCHIOSTOMA-MOUTH

Mouth is large oval median aperture situated below the rostrum tip of the trunk. It is bordered by a frill-like membrane , the oral hood.

BRANCHIOSTOMA-ALIMENTARY CANAL

The alimentary canal is a straight and complete tube from mouth to anus.It is lined by ciliated epithelium and diameter varies in different parts.

BRANCHIOSTOMA-DIGESTIVE SYSTEM

The digestive system consists of an alimentary canal and digestive glands.

BRANCHIOSTOMA-ATRIUM

The atrial is large body space other than coelom. It is formed by a pair of metapleural folds,on either side above the gill-slits of embryo,growing downwards and united ventrally by a transverse folds. In branchiostoma the gill-slits open intothe atrium and not directly to the exterior as in other chordates.The atrium projects into each dorsal coelomic canal on either side of pharynx forming the brown funnel or atriocoelomic canal whose function is unknown. The atrium projects the delicate pharyngeal region.

BRANCHIOSTOMA-COELOM

The body cavity in branchiostoma is true coelom lined with somatic and splanchnic layers of mesoderm and filled with a lymph like coelomic fluid. It is enterocoelic in origin. The pair of dorsal longitudinal coelomic canals are present. A mid ventral longitudinal subendostylar coelomic canals running through the primary gill bars.Besides small coelomic spaces are also present within the gonads and around the midgut or liver diverticulum.

BRANCHIOSTOMA- OTHER SKELETAL STRUCTURES

These include
a) fin-ray boxes
b) oral ring
c) gill-rods
d) endostylar plates
e) tough fibrous connective tissue.

BRANCHIOSTOMA-NOTOCHORD

The axial endoskeleton notochord are called chorda dorsalis. It is elongated ,narrow ,cylindrical rod-like structures from tip to tail. It lies mid-dorsally at gut and nerve cord. The notochord  is surrounded by a laminated fibrous connective tissue notochordal sheath covered by a thin elastic membrane. The elasticity of notochord bending body movement but its rigidly prevents shortening of body when longitudinal muscles contract locomotion.

BRANCHIOSTOMA-SKELETON

has no skeleton. A number of endoskeletal structures are present. They are neither bony nor cartilagnious.

BRANCHIOSTOMA-MUSCULATURE

The muscles lying beneath the skin form the part of body wall. They are thick in dorso-lateral regions.They length of body present ina series of v-shaped myotomes or myomeres. The pointed apex of the vis anteriorly. In B.lanceolatum 60-62 myotomes are present. Myotomes are enveloped in a thin connective tissue layer . Due to their peculiar arragement several myotomes and myocommata . The myotomes help in lateral undulating movements of body as they can be twisted .Their contractions help in driving water of atrival cavity out of the atriopore.

BRANCHIOSTOMA-PERITONEUM

The muscle layer is lined internally by a thin layer of parietal paritoneum. It is not continuous in the pharyngeal region .

BRANCHIOSTOMA-MYOTOMES

The lateral side of body a series of muscle bands,called myotomes or myomeres. Between mouth and atriopore present on either side below the myotomes gonads.

BRANCHIOSTOMA-BODY WALL

The body wall includes from outside- A thin delicate and transparent skin .
A well-developed musculature.
A parietal peritoneum.

BRANCHIOSTOMA-MYOTOMES AND GONADS

On each lateral side of body a series of v-shaped muscle band called myotomes or myomeres  present. Between mouth and atriopore, present on eitherside below the myotomes .A series of gonads also present..

BRANCHIOSTOMA-FINS AND FOLDS

Branchiostoma bears three fins dorsal, caudal and ventral. The dorsal fins runs as a low, mid-dorsal fold along the entire length of trunk. It is contianuous behind with a much broader caudal fin around the tail. The ventral fin runs mid-ventrally from caudal fin upto atriopore. It is slightly wider than the dorsal fin. Fin rays are lacking in the caudal fin.The structure of fins and fin-ray boxes is different from that in fishes. Paired fins are absent.The are two hollow membranous metapleural folds.The two folds are connected by a horizontal fold of body wall, called epipleur, which forms the floor of the body.