| Habits and Habitat | Solitary / Colonial, Tubiculous, Worm like marine animals with segmented body. Moves in its burrow by cilia covering its body surface. |
| External Morphology | Cylindrical body: -Proboscis – conical – thick muscular wall and hollow within. -Collar – funnel like – completely surrounds and conceals the proboscis stalk. -Trunk – cylindrical and further divided into Branchiogenital region, Hepatic region, Abdominal region. Bilateral Symmetry, Triploblastic, Coelomates. Notochord confined to head region. |
| Body Wall | Composed of Epidermis, Musculature and Peritoneum. Epidermis: -Single layer of mostly tall, slender, columnar and ciliated cells. -Gland cells secreting mucus are Goblet gland cells, Reticulate gland cells, Granular gland cells. -Gland cells are most abundant in the collar region. -Below the Epidermis is a thick nervous layer consisting of a network of nerve cells and nerve fibres. -Below the neural layer is a basement membrane Musculature: -Smooth, weak and mostly longitudinal. Peritoneum: -Coelom is lined by parietal coelomic epithelium which covers the inner surface of the body wall musculature. Functions: Shields the soft internal organs from mechanical injuries. Mucus produced by epidermal gland cells adheres sand particles for lining the burrow in which the worm lives. Foul smell of mucus is protective. Neuro-sensory cells serve to receive external stimuli. Musculature helps in body movements. |
| Coelom | Spacious coelom lined by coelomic epithelium. Coelomic cavities of proboscis, collar and trunk are completely separated from one another. Proboscis and collar coelom communicate with the exterior and are largely filled with sea water which keeps them turgid. Trunk coelom is filled with a watery coelomic fluid containing amoeboid coelomocytes (which originate from the coelomic epithelium). They secrete a membrane around any foreign body that may invade the animal. |
| Endoskeleton | No definite endoskeleton of bone or cartilage. 4 types of stiff structure: Buccal Diverticulum: -From the roof of buccal cavity, short, stiff, thick walled, hollow projection extends forward through the proboscis stalk into the proboscis coelom. -Different from true notochord of other vertebrates. Proboscis Skeleton: Y shaped chitinous structure formed by thickening of basement membrane. Branchial Skeleton: -Formed by thickening of the basement membrane. -Lie in the wall of the pharynx and support U shaped gill slits that perforate it. Pygochord: Rod-like thickening that supports the post hepatic region. |
| Digestive System | Gut is straight or U shaped. Digestive tract is complete between mouth and anus. It is supported throughout its length by dorsal and ventral mesenteries. Walls have ciliated epithelium. Alimentary canal: Mouth, Buccal Cavity, Pharynx, Oesophagus, Intestine, Anus. Intestinal wall is in close contact with intestinal sacculations. Undigested food along with sand and silt pass out the anus as castings. Feeding: Ciliary feeder — food comprises of microscopic organisms and organic particles present in water and bottom sand. Lateral cilia lying the gill slits set up a current of water which enters through the mouth, takes it course through buccal cavity, pharynx, gill slits and branchial sacs, and leaves through gill pores — forms the respiratory cum food current. |
| Respiration System | Partially open circulation, Respiration through gills. Respiratory apparatus comprises Branchial portion of Pharynx bearing gill slits. Number of gill slits varies and increases as the animal grows older. Gill slits open into branchial sacs that open out through gill pores. |
| Circulatory System | Blood: colourless fluid, containing few white corpuscles which are possibly detached endothelial cells. Respiratory pigment not known. Central Sinus and Heart Vesicle: Central Sinus is a small elongated non-contractile sinus situated in the proboscis just above the buccal diverticulum. Heart vesicle has muscular ventral wall which contracts rhythmically, producing pulsations in the central sinus, that help in circulation in the blood. |
| Neural System | Primitive type neural system resembling coelenterates and echinoderms. Throughout the body, a plexus or a layer of nerve cells and nerve fibres lie just below the epidermis. |
| Excretory System | Excretory organ is Glomerulus which projects into the Proboscis Coelom. From Glomerulus, the excretory substances pass on into the proboscis coelom and finally to the exterior through the proboscis pore. |
| Reproductive System | Sexes are separate. Males and females look similar externally except for difference in colour of ripe gonads. Testes and Ovaries are similar — sac-like bodies occurring in several longitudinal rows in the genital wings, on either side of the alimentary canal. Ovaries are small with pro yolk content. Fertilisation is external. |
Affinities:
Peculiar anatomical organisation of Hemichordates makes their systematic position uncertain and controversial. They were initially classified as a sub phylum under chordates, but now are treated as an independent phylum of invertebrates.
Chordates
Common Features: Pharyngeal gill slits, Nerve Cord, Notochord.
Urochordate: Structure and function of pharynx, development of central part of neural system.
Cephalochordate: structure and function of branchial apparatus, arrangement of coelomic sacs and developmental sequence.
However, the hemichordates are no longer included under chordates, because they do not possess certain typical chordate characters:
Absence of true notochord.
-It is short, confined to proboscis and without any supporting function.
-It is ventral to the main blood vessel and not covered by a sheath.
-Hollow and lined by epithelial cells, instead of being solid and being made of vacuolated cells.
Hence, the notochord is now called Stomochord or Buccal Diverticulum.
Neural System is distinctly of the invertebrate type.
-Intra-epidermal in position.
-Ventral nerve cord and circum-enteric nerve ring which are absent in chordates.
-Dorsal tubular nerve cord is confined to collar region only.
Gill slits are numerous and dorsal. In higher chordates, they are 5 to 7 and lateral.
Lack metameric segmentation, cephalisation, paired appendages, post anal tail, living endoskeleton.
Annelids
Worm like and segmented body.
Burrowing habit, ingest mud which is passed out as castings.
Dorsal position of heart.
Tornaria larva of Balanoglossus shows several structural resemblances with Trochophore larva of Annelid -pelagic, ciliated, well developed alimentary canals.
However, annelids are not deuterosomes (blastopore develops into mouth in annelids, it becomes anus
in Balanoglossus)
Possibly, a convergent evolution due to similar habits and habitat.
Echinoderms
Blastopore, Ciliated bands in larvae.
Proteins and phosphagens present in hemichordates closely resemble echinoderms.
Abilities of regeneration.
Heart vesicle and glomerulus of Balanglossus have functions analogous to Dorsal Sac and axial gland of echinoderms — combine both vascular and excretory functions.
Neural system is poorly developed and forms epidermal nerve plexus.
Similarities of larvae: Tornaria was regarded as echinoderm larvae for a long time.