Life and Diversity of Animals – Nonchordates (Arthropoda to Hemichordata) BZO2T03

Syllabus topics (RTMNU)

• 1.1 Arthropoda: General characters of the phylum and classification up to classes with suitable examples
• 1.2 Cockroach: Digestive system, Male and Female reproductive system
• 1.3 Insects as Vectors : Mosquito, Housefly, Tse-Tse fly
• 1.4 Study of crustacean larvae: Nauplius, Zoea and Megalopa

1.1 Arthropoda – general characters and classification

Arthropoda (Gk. arthron = joint, podos = foot) is the largest phylum of the animal kingdom; more than three-fourths of all known animal species are arthropods, most of them insects.

General characters

• Triploblastic, bilaterally symmetrical, segmented (metameric) animals with organ-system level of organisation.
• Body covered by a hard chitinous exoskeleton (cuticle) secreted by the epidermis; it is shed periodically by moulting (ecdysis).
• Jointed appendages – one pair per segment in primitive forms.
• Body divided into regions (tagmatisation): head, thorax, abdomen, or cephalothorax and abdomen.
• Coelom is reduced; the main body cavity is a blood-filled haemocoel.
• Open circulatory system; dorsal tubular heart with ostia.
• Respiration by body surface, gills (aquatic), trachea (insects), book lungs (scorpion, spider) or book gills (Limulus).
• Excretion by Malpighian tubules (insects, arachnids) or green glands / antennary glands (crustaceans).
• Nervous system: dorsal brain + double ventral nerve cord with segmental ganglia. Sense organs: antennae, simple eyes (ocelli) and compound eyes.
• Muscles are striated and in bundles; no cilia anywhere in the body.
• Sexes usually separate (dioecious), often with sexual dimorphism; fertilisation mostly internal; development direct or indirect with larval stages and metamorphosis.

Classification up to classes (as commonly followed in Indian university texts):

1.2 Cockroach (Periplaneta americana) – digestive and reproductive systems

A. Digestive system – the alimentary canal is a long coiled tube from mouth to anus, divided into foregut, midgut and hindgut.

• Mouth parts (biting and chewing type): labrum (upper lip), a pair of mandibles, a pair of first maxillae, labium (lower lip, formed by fused second maxillae) and hypopharynx (tongue-like).
• Foregut (stomodaeum) – lined by cuticle: pre-oral cavity → mouth → pharynx → oesophagus → crop (large sac for storage) → gizzard (proventriculus) with 6 chitinous teeth for grinding food; a stomodaeal valve controls entry into the midgut.
• Midgut (mesenteron) – the true stomach, lined by glandular endoderm; at the junction of fore- and midgut lie 6–8 finger-like hepatic (gastric) caeca that secrete enzymes. A thin peritrophic membrane protects the midgut lining. Digestion and absorption occur here.
• Hindgut (proctodaeum) – lined by cuticle: ileum → colon → rectum → anus. About 100–150 yellow Malpighian tubules open at the junction of midgut and hindgut and remove nitrogenous waste (uric acid). The rectum reabsorbs water.
• Salivary glands: one pair, lying on either side of the crop; each has glandular lobes plus a salivary receptacle (reservoir). Ducts unite to open at the base of the hypopharynx. Saliva contains amylase.

Diagram in words: draw a long tube; label crop (balloon-like), gizzard (small rounded), ring of hepatic caeca, narrow midgut, Malpighian tubules as a bunch of threads at the midgut–hindgut junction, then ileum, coiled colon and swollen rectum.

B. Male reproductive system (abdominal segments 4–6)

• A pair of testes in the 4th–6th abdominal segments, each with 3–4 lobes, embedded in fat body.
• A thin vas deferens from each testis opens into the ejaculatory duct through the seminal vesicles.
• Mushroom gland – a large accessory gland at the 6th–7th segments; its peripheral tubules (utriculi majores) and central tubules (utriculi breviores) secrete material for the spermatophore; seminal vesicles store sperm.
• Conglobate gland (phallic gland) – opens near the gonopore; helps form the outer layer of the spermatophore.
• Ejaculatory duct opens by the male gonopore on the ventral side of the anus; external genitalia = phallomeres (left, right and ventral) – chitinous, asymmetrical.

C. Female reproductive system

• A pair of ovaries in the 2nd–6th abdominal segments; each ovary has 8 ovarioles (ovarian tubules) with developing eggs in a chain.
• Ovarioles of each side join to form an oviduct; the two oviducts unite into a common oviduct (vagina) opening into the genital chamber (genital pouch).
• Spermathecae – a pair in the 6th segment, store sperm received during copulation; open into the genital chamber.
• Collaterial glands – a pair of branched glands; their secretion forms the hard egg case.
• Gonapophyses (3 pairs) act as ovipositor. Fertilised eggs are packed in an ootheca (about 16 eggs, in two rows) which is dropped in a safe place. Development is paurometabolous (egg → nymph → adult; nymph moults about 13 times).

1.3 Insects as vectors – Mosquito, Housefly, Tse-tse fly

A vector is an organism (often an insect) that carries a pathogen from an infected host to a healthy host. In a biological vector the pathogen multiplies or develops inside it; a mechanical vector only carries it on its body.

• Mosquito: only females suck blood (needed for egg development); mouth parts piercing–sucking. Anopheles rests at an angle to the surface; Culex rests parallel. Control: drain stagnant water, larvivorous fish (Gambusia, guppy), oil films on water, mosquito nets, repellents, insecticide spraying.
• Housefly: sits on faeces and garbage and then on food. Pathogens stick to its hairy legs and sticky pads (pulvilli), are vomited ("vomit drop") or passed in faeces onto food. Control: cleanliness, covered food and garbage, fly traps, insecticides.
• Tse-tse fly: found in tropical Africa; both sexes suck blood. The trypanosome multiplies in the fly's gut and salivary glands and enters man at the bite. Symptoms: fever, swollen lymph glands, lethargy, sleepiness, coma. Control: clearing bush near villages, traps, insecticides, sterile-male technique.

1.4 Crustacean larvae – Nauplius, Zoea and Megalopa

• Nauplius: the first, simplest larva of crustaceans (e.g. Cyclops, barnacles, Penaeus). Body unsegmented, oval/pear-shaped; a single median naupliar eye; three pairs of appendages – antennules (uniramous), antennae (biramous) and mandibles (biramous) – used for swimming. Labrum covers the mouth. Free-swimming and planktonic.
• Zoea: typical of crabs (e.g. Carcinus) and found after nauplius in many decapods. Has a large cephalothorax with a long dorsal (rostral) spine, an anterior rostral spine and often two lateral spines (for floating and defence); a pair of stalked compound eyes; long segmented abdomen ending in a forked telson. Swims with maxillipeds.
• Megalopa: the last larval stage of crabs, after zoea. Crab-like broad cephalothorax, large stalked compound eyes, all thoracic legs (with chelae) developed, but the abdomen is still extended and bears pleopods for swimming. It settles at the bottom, folds its abdomen under the cephalothorax and moults into a young crab.

Likely exam questions

1. (Long, 8 marks) Give the general characters of phylum Arthropoda and classify it up to classes with examples.
2. (Long, 8 marks) Describe the digestive system of cockroach with a labelled diagram. / Describe the male and female reproductive systems of cockroach.
3. (Short, 4 marks) Explain the role of housefly as a mechanical vector.
4. (Short, 4 marks) Write a note on the tse-tse fly and sleeping sickness.
5. (Short, 4 marks) Describe the Zoea larva (or: Nauplius / Megalopa) with a labelled sketch.

Syllabus topics (RTMNU)

• 2.1 Mollusca: General characters and classification up to classes
• 2.2 Pila: Respiratory system and Mechanism of Aquatic and Pulmonary respiration.
• 2.3 Economic importance of Mollusca: Edible oyster farming, pearl forming mollusc, types of pearl culture
• 2.4 Molluscan larva: Glochidium and Veliger

2.1 Mollusca – general characters and classification

Mollusca (L. mollis = soft) is the second largest phylum. Molluscs are soft-bodied animals, usually protected by a calcareous shell.

• Triploblastic, coelomate (coelom reduced to pericardial cavity, gonocoel and kidney cavities), unsegmented, bilaterally symmetrical; many gastropods become asymmetrical because of torsion.
• Body has head, ventral muscular foot and dorsal visceral mass.
• A fold of body wall, the mantle (pallium), covers the visceral mass and secretes the shell (CaCO₃). The space between mantle and visceral mass is the mantle cavity.
• Mouth cavity usually with a rasping tongue, the radula (absent in Bivalvia).
• Respiration by gills called ctenidia, or by a pulmonary sac (lung) in land snails.
• Circulatory system usually open (closed in cephalopods); heart in a pericardium with auricles and a ventricle; blood contains haemocyanin (copper pigment, blue).
• Excretion by one or two kidneys (organs of Bojanus / metanephridia).
• Nervous system of paired ganglia (cerebral, pleural, pedal, visceral) with connectives; sense organs: eyes, tentacles, osphradium (tests water) and statocysts.
• Sexes separate or hermaphrodite; development direct or with trochophore and veliger larvae.

2.2 Pila globosa (apple snail) – respiratory system and mechanism of respiration

Pila is an amphibious freshwater snail. It lives in ponds and paddy fields and can breathe both in water and in air; in summer it buries itself in mud and aestivates. Hence it has two kinds of respiratory organs.

Respiratory organs

• Ctenidium (gill) – a single gill on the right side of the mantle cavity, attached to the mantle roof. It is monopectinate: one row of triangular gill lamellae (leaflets) hanging from an axis, richly supplied with blood. Used in aquatic respiration.
• Pulmonary sac (lung) – a large closed sac on the left side of the mantle cavity, formed from the mantle; opens into the mantle cavity by a pulmonary aperture; its wall is highly vascular. Used in aerial (pulmonary) respiration.
• Nuchal lobes (pseudepipodia) – two fleshy folds of the mantle at the sides of the head. The left nuchal lobe can be rolled into a long siphon for taking in air; the right lobe guides water out.
• Epitaenia – a ridge on the floor of the mantle cavity that divides it into a left pulmonary chamber (receiving the opening of the lung) and a right branchial chamber (containing the ctenidium).

Mechanism of aquatic respiration

1. The snail is submerged; the head and foot are extended out of the shell.
2. Water enters the mantle cavity on the left side, guided by the left nuchal lobe, driven by the beating of cilia on the mantle and gill.
3. It crosses over the epitaenia into the right branchial chamber and flows over the gill lamellae; O₂ diffuses into the blood and CO₂ diffuses out.
4. Deoxygenated water leaves on the right side, guided by the right nuchal lobe. The direction of flow (left to right) is maintained by cilia.

Mechanism of pulmonary (aerial) respiration

1. When the water has little oxygen, Pila comes to the surface (or stays partly out of water).
2. The left nuchal lobe is rolled into a siphon tube whose tip projects above the water surface.
3. By contraction and relaxation of the body (pumping movements of the head–foot / mantle), air is drawn through the siphon into the mantle cavity and into the pulmonary sac through its aperture.
4. Gas exchange takes place across the vascular wall of the lung; stale air is expelled by the reverse movement.
5. During aestivation the operculum closes the shell mouth and slow aerial respiration continues through the lung.

2.3 Economic importance – edible oyster farming, pearl-forming molluscs, pearl culture

General importance: food (oysters, mussels, clams, squids); pearls and mother-of-pearl buttons; shells for lime, ornaments and conch (Turbinella); harmful forms – shipworm Teredo (bores wood), garden slugs and Achatina (crop pests), snails as intermediate hosts of liver fluke.

A. Edible oyster farming (oyster culture)

• Species in India: Crassostrea madrasensis (Indian backwater oyster), C. gryphoides, C. cucullata. Rich in protein, glycogen, minerals (zinc, iron) and vitamins.
• Site selection: sheltered bays, estuaries and backwaters with salinity about 15–30 ppt, plenty of plankton and little silt.
• Spat collection: free-swimming larvae settle on hard surfaces called cultch – lime-coated tiles, oyster shells strung on ropes, bamboo poles.
• Methods of culture: (1) Bottom culture – spat spread on the sea floor; (2) Rack and string (ren) method – shells with spat strung on ropes hung from racks in water; (3) Raft method – strings hung from floating rafts; (4) Stake / pole method.
• Oysters reach marketable size (7–8 cm) in about 8–12 months; they are harvested, cleaned (depuration in clean seawater to remove bacteria) and sold. CMFRI (Kochi) developed the technology in India.

B. Pearl-forming molluscs

• Marine pearl oysters: Pinctada fucata (the main Indian pearl oyster, Gulf of Mannar), P. margaritifera (black-lip), P. maxima (gold/silver-lip).
• Freshwater pearl mussels: Lamellidens marginalis, L. corrianus, Parreysia (India); Hyriopsis (China, Japan).
• Formation of pearl: when a foreign particle (sand grain, parasite) enters between the mantle and shell, mantle epithelial cells surround it to form a pearl sac, which secretes concentric layers of nacre (aragonite crystals of CaCO₃ bound by an organic protein, conchiolin). Layers build up into a pearl.

C. Types of pearl culture

• Natural pearl – formed without human help; rare and costly.
• Cultured pearl – nucleated (marine, Mikimoto method, Japan): a round nucleus (bead of freshwater mussel shell) together with a small piece of mantle tissue (graft) from a donor oyster is surgically implanted into the gonad of a host pearl oyster. Oysters are kept in cages on rafts for 1–2 years; then pearls are harvested.
• Tissue-nucleated / non-nucleated (freshwater) culture: only small mantle grafts are inserted into the mantle of freshwater mussels; each graft forms a pearl sac. Many pearls per mussel (common in China and India).
• Mabe (image / half) pearls: a hemispherical or shaped nucleus is glued to the inner surface of the shell; nacre covers it, producing a half-pearl or designer pearl (e.g. image of a god). CIFA, Bhubaneswar developed freshwater pearl culture in India.

2.4 Molluscan larvae – Glochidium and Veliger

• Glochidium: the parasitic larva of freshwater mussels (Unionidae, e.g. Unio, Anodonta, Lamellidens). Tiny (0.1–0.4 mm) with a bivalved shell; each valve often bears a hooked tooth at its free edge; a long sticky larval thread (byssus); a single adductor muscle and sensory hairs. Larvae develop in the gills (marsupium) of the mother, are released, and attach to the gills or fins of fish, where they live as ectoparasites in a cyst for a few weeks, then drop off as young mussels. Significance: helps dispersal of a sedentary animal and avoids being washed downstream.
• Veliger: the characteristic larva of most marine gastropods and bivalves; develops from the trochophore. Has a velum – a ciliated, often bilobed, swimming and feeding organ (formed from the prototroch); a developing shell, foot, mantle and visceral mass; statocysts and eyes may appear. In gastropods torsion takes place at the veliger stage. The veliger sinks, loses the velum and becomes a young snail/bivalve.

Likely exam questions

1. (Long, 8 marks) Give the general characters of Mollusca and classify it up to classes with examples.
2. (Long, 8 marks) Describe the respiratory organs of Pila and explain the mechanism of aquatic and pulmonary respiration.
3. (Short, 4 marks) Write a note on edible oyster farming.
4. (Short, 4 marks) How is a pearl formed? Describe the types of pearl culture.
5. (Short, 4 marks) Describe the Glochidium larva / Veliger larva with a sketch.

Syllabus topics (RTMNU)

• 3.1 Echinodermata: General characters and classification up to classes
• 3.2 Asterias: External morphology, Straight and Crossed type of Pedicellariae.
• 3.3 Water vascular system and locomotion in Asterias
• 3.4 Echinoderm larva: Bipinnaria and Auricularia

3.1 Echinodermata – general characters and classification

Echinodermata (Gk. echinos = spiny, derma = skin) are spiny-skinned, exclusively marine animals.

• Triploblastic, coelomate, deuterostome animals (like chordates); enterocoelous coelom.
• Adults show pentamerous radial symmetry (body in five parts), but larvae are bilaterally symmetrical – the radial symmetry is secondary.
• No distinct head; body has an oral and an aboral surface; no segmentation.
• Endoskeleton of calcareous plates (ossicles) of mesodermal origin, often bearing spines.
• A unique water vascular (ambulacral) system with tube feet (podia) for locomotion, food capture and respiration.
• Body surface may bear pedicellariae (small pincers) and dermal branchiae (papulae) for respiration.
• Digestive system usually complete; no special excretory organs (amoebocytes carry wastes).
• Haemal (blood-lacunar) system and perihaemal system present; no heart.
• Nervous system without brain: a circumoral nerve ring and radial nerves.
• Sexes usually separate, no sexual dimorphism; fertilisation external; development indirect with free-swimming bilateral larvae (bipinnaria, auricularia, pluteus, doliolaria).
• Great power of regeneration and autotomy.

3.2 Asterias (starfish) – external morphology and pedicellariae

External morphology

• Star-shaped, flattened body, 10–20 cm across, with a central disc and usually five tapering arms (rays). Colour orange, red, purple or brown.
• Oral (lower) surface: the mouth lies in the centre surrounded by a soft peristomial membrane and five pairs of oral spines (mouth papillae). From the mouth an ambulacral groove runs along the middle of each arm to its tip; each groove has 2–4 rows of tube feet with terminal suckers and is guarded by movable ambulacral spines.
• At the tip of each arm is a small sensory terminal tentacle with a red eye-spot (optic cushion) at its base.
• Aboral (upper) surface: rough, with many short calcareous spines; between spines are dermal branchiae (papulae) – thin respiratory outgrowths; and pedicellariae.
• The small anus lies near the centre of the aboral disc.
• A sieve-like plate, the madreporite, lies on the aboral surface of the disc between two arms (interradial). These two arms are the bivium; the other three are the trivium.

Pedicellariae – minute, stalked or sessile, jaw-like calcareous pincers on the body surface. Functions: keep the body surface clean by removing debris and larvae of other animals that try to settle; protect the delicate dermal branchiae; help capture small prey.

3.3 Water vascular system and locomotion in Asterias

The water vascular (ambulacral) system is a system of sea-water-filled canals derived from the coelom (hydrocoel). It works hydraulically.

1. Madreporite – a porous calcareous plate on the aboral surface; sea water enters through its pores and grooves.
2. Stone canal – an S-shaped vertical canal, its wall stiffened by calcareous rings, leading from the madreporite down to the oral side.
3. Ring canal – a pentagonal canal round the oesophagus (mouth).
4. Tiedemann's bodies – 9 small glandular bodies on the inner side of the ring canal (interradial; not at the position of the stone canal); produce amoeboid cells (coelomocytes) for the fluid.
5. Polian vesicles – absent in Asterias (present in some other starfishes); act as reservoirs where present.
6. Radial canals – five, one into each arm, running along the floor of the ambulacral groove above the ambulacral ossicles.
7. Lateral canals – short side branches of each radial canal, alternately long and short, each with a valve to prevent backflow; each ends in a tube foot.
8. Tube foot (podium) – each has an upper bulb-like ampulla inside the arm (above the ambulacral ossicles, passing through the ambulacral pores) and a lower muscular tubular podium ending in a sucker.

Mechanism of locomotion

1. Extension: muscles in the wall of the ampulla contract; the valve in the lateral canal closes, so water is forced into the podium, which elongates and stretches forward in the direction of movement.
2. Attachment: the sucker presses on the substratum; muscles raise its centre creating a vacuum, and adhesive secretion helps it hold firmly.
3. Pulling (contraction): longitudinal muscles of the podium contract; water goes back into the ampulla; the podium shortens and swings backward, pulling the body forward.
4. Release: the sucker releases (de-adhesive secretion, relaxation), the tube foot is lifted and the cycle repeats.

Thousands of tube feet work in co-ordination (controlled by the radial nerves and nerve ring); one arm takes the lead. Movement is slow, about 15 cm per minute. Tube feet are also used to open bivalve shells while feeding, to cling to rocks and for respiration.

3.4 Echinoderm larvae – Bipinnaria and Auricularia

• Bipinnaria (larva of Asteroidea, e.g. Asterias): free-swimming, bilaterally symmetrical, transparent planktonic larva. It develops from a dipleurula-like stage. Its ciliated band becomes divided into a pre-oral loop and a post-oral loop that run along several paired lobes or arms (pre-oral, antero-dorsal, postero-dorsal, post-oral, postero-lateral). The gut is complete: mouth, oesophagus, stomach, intestine and anus. Coelomic sacs present. It feeds on plankton. In Asterias it changes into a brachiolaria larva (with 3 adhesive brachiolar arms and a sucker) which attaches and metamorphoses into a young starfish.
• Auricularia (larva of Holothuroidea, sea cucumber): small, transparent, bilaterally symmetrical larva with a single, wavy, longitudinal ciliated band that forms ear-like lateral lobes (hence the name, L. auricula = little ear). It has a pre-oral lobe, ventral mouth, curved gut and anus; hydrocoel develops. It changes into a barrel-shaped doliolaria (pupa) larva with ciliated rings, then into a young sea cucumber. The auricularia resembles the tornaria larva of Balanoglossus – evidence of echinoderm–hemichordate relationship.

Likely exam questions

1. (Long, 8 marks) Give the general characters of Echinodermata and classify it up to classes with examples.
2. (Long, 8 marks) Describe the water vascular system of Asterias and explain how it helps in locomotion.
3. (Short, 4 marks) Describe the external morphology of Asterias.
4. (Short, 4 marks) Distinguish between straight and crossed pedicellariae.
5. (Short, 4 marks) Describe the Bipinnaria larva / Auricularia larva with a sketch.

Syllabus topics (RTMNU)

• 4.1 Hemichordata: General characters and Phylogeny
• 4.2 Balanoglossus: External features and digestive system
• 4.3 Pre-Larval Development in Balanoglossus, Tornaria larva
• 4.4 Affinities of Balanoglossus with Annelida, Echinodermata and Chordata

4.1 Hemichordata – general characters and phylogeny

Hemichordata (Gk. hemi = half, chorde = string) are small, soft-bodied, worm-like marine animals, mostly living in U-shaped burrows (acorn worms) or in tubes (pterobranchs).

• Triploblastic, bilaterally symmetrical, enterocoelous deuterostomes; unsegmented.
• Body divided into three regions – proboscis (protosome), collar (mesosome) and trunk (metasome) – each with its own coelom: protocoel (single), mesocoel and metacoel (paired). The body is thus tricoelomate.
• Body surface ciliated and glandular (mucus).
• A hollow outgrowth from the roof of the buccal cavity into the proboscis, the stomochord (buccal diverticulum), was once regarded as a notochord; today it is not considered homologous to it.
• Pharyngeal gill slits present (many pairs in enteropneusts; one pair or none in pterobranchs) – a chordate feature.
• Digestive tract complete and straight (U-shaped in pterobranchs); feeding by ciliary-mucous method.
• Open circulatory system with a dorsal and a ventral vessel; a contractile heart vesicle and a glomerulus (excretory) in the proboscis.
• Nervous system mainly intra-epidermal; a dorsal nerve cord in the collar is hollow (with neurocoel) in some species.
• Sexes usually separate; fertilisation external; development direct or indirect through a tornaria larva.

Classes: (1) Enteropneusta – acorn/tongue worms, solitary, free, many gill slits (Balanoglossus, Saccoglossus, Ptychodera). (2) Pterobranchia – small, colonial, tube-dwelling, collar with tentaculated arms, U-shaped gut (Cephalodiscus, Rhabdopleura).

Phylogeny (systematic position)

• Bateson (1885) placed Hemichordata under Chordata as a subphylum because of three chordate-like characters: the "notochord" (stomochord), pharyngeal gill slits and a dorsal hollow nerve cord in the collar.
• Later workers (Van der Horst, Hyman 1959, and modern authors) argued that the stomochord is only a buccal diverticulum, not a true notochord; the nerve cord is mostly not tubular; and there is no post-anal tail or myotomes. Therefore Hemichordata is now treated as a separate phylum of non-chordates.
• They share many features with Echinodermata (deuterostome, enterocoel, three-part coelom, similar larva). Both form the group Ambulacraria, which is the sister group of Chordata. Hemichordates are thus a link between invertebrates and chordates.

4.2 Balanoglossus (tongue worm) – external features and digestive system

External features

• Soft, elongated, worm-like, cylindrical; length from a few cm to over 1 m (commonly 10–50 cm). Yellowish/orange with a peculiar odour (bromine compounds). Lives in U-shaped burrows in sand on sea shores; Balanoglossus occurs on Indian coasts.
• Proboscis (protosome) – short, conical, muscular, acorn-shaped anterior region used for burrowing and collecting food; joined to the collar by a narrow proboscis stalk. Its cavity (protocoel) opens to the outside by a proboscis pore.
• Collar (mesosome) – short cylindrical region behind the proboscis; its anterior edge (collarette) surrounds the stalk. The mouth lies ventrally between the proboscis stalk and the collar. Collar coelom opens by a pair of collar pores.
• Trunk (metasome) – the longest part, divided into:
  • Branchio-genital region – anterior part with a mid-dorsal row of gill pores on each side and lateral genital wings (ridges) containing the gonads.
  • Hepatic region – with dorsal, brownish/greenish sacculations, the hepatic caeca.
  • Post-hepatic (abdominal / caudal) region – tapering hind part ending in the terminal anus.

Digestive system – a straight tube from mouth to anus.

1. Mouth – wide, permanently open, ventral, at the junction of proboscis and collar.
2. Buccal cavity – lies inside the collar. From its roof a hollow tube, the stomochord (buccal diverticulum), projects forward into the proboscis stalk.
3. Pharynx – in the branchio-genital region; divided by lateral ridges (parabranchial ridges) into a dorsal branchial part with U-shaped gill slits (opening into branchial sacs and gill pores), and a ventral digestive (food) part through which food passes.
4. Oesophagus – behind the pharynx; may have a few small dorsal pores (oesophageal pores) that let out excess water.
5. Intestine – long; its anterior (hepatic) part bears the hepatic caeca, which secrete digestive enzymes and absorb food. The posterior part runs straight to the anus.

Food and feeding: Balanoglossus is a ciliary-mucous feeder and a detritus/sand feeder. Organic particles and microorganisms trapped in mucus on the proboscis are carried by cilia backwards to the mouth. Water leaves through the gill slits; food in mucous strings passes through the ventral part of pharynx to the intestine. Sand swallowed while burrowing comes out as coiled castings at the burrow opening.

4.3 Pre-larval development in Balanoglossus and the Tornaria larva

Pre-larval development (from egg to larva)

• Fertilisation: sexes separate; eggs and sperm are shed into sea water at night; fertilisation is external.
• Egg: small, with little yolk (microlecithal), surrounded by a membrane.
• Cleavage: holoblastic, equal and radial (like echinoderms), forming 2, 4, 8, 16 … cells.
• Blastula: a hollow ball of cells – coeloblastula with a blastocoel; it becomes flattened on one side.
• Gastrula: formed by invagination of the flattened side; the opening is the blastopore, which later closes; the anus forms at or near its site (deuterostome).
• Mesoderm and coelom: formed by enterocoely – pouches from the archenteron give the protocoel, and later mesocoels and metacoels.
• The embryo becomes ciliated, the mouth (stomodaeum) breaks through, and it hatches as a free-swimming tornaria larva (indirect development, e.g. B. clavigerus; some species such as Saccoglossus have direct development).

Tornaria larva

• Discovered by J. Müller (1850), who took it for an echinoderm larva; Metschnikoff (1869) showed it belongs to Balanoglossus.
• Small (about 1–3 mm), transparent, oval and pelagic.
• Apical plate at the anterior end with a pair of eye-spots and a tuft of sensory cilia.
• Ciliary bands: a sinuous circumoral (neurotroch) band divided into pre-oral and post-oral parts (used in feeding), and a strong ring of cilia round the hind end, the telotroch (perianal band), for swimming.
• Complete, curved digestive tract: mouth, oesophagus, stomach, intestine, anus.
• A protocoel opening to the outside by a dorsal hydropore (proboscis pore); a muscle strand runs from the protocoel to the apical plate. Mesocoels and metacoels appear later.
• Metamorphosis: the larva elongates, loses its ciliary bands and sinks; the region in front of the collar groove becomes the proboscis, followed by collar and trunk; gill slits appear and it becomes a young worm.

4.4 Affinities of Balanoglossus

A. With Annelida

• Worm-like, elongated body; burrowing habit, swallowing sand and leaving castings like earthworms.
• Dorsal blood vessel with forward flow and ventral vessel with backward flow, as in annelids.
• A ventral nerve cord in the trunk.
• The tornaria larva resembles the trochophore larva of annelids (apical plate with eye-spots, ciliary bands, telotroch).
• Differences: no segmentation, no parapodia/setae, no nephridia; coelom enterocoelous (schizocoelous in annelids); gill slits present. Hence the resemblance is only superficial.

B. With Echinodermata (close relationship)

• Both are deuterostomes with enterocoelous coelom; coelom in three parts (protocoel, mesocoel, metacoel ↔ axocoel, hydrocoel, somatocoel).
• Tornaria is strikingly similar to bipinnaria and especially auricularia larvae – both have ciliary bands with pre-oral and post-oral loops, a complete gut and a protocoel opening by a dorsal pore (hydropore).
• Proboscis pore of Balanoglossus is comparable to the madreporite/hydropore of echinoderms.
• Nervous system mostly intra-epidermal (sub-epidermal plexus) in both.
• Both creatine phosphate and arginine phosphate occur in hemichordates and some echinoderms.
• Differences: echinoderms are radially symmetrical as adults, have a water vascular system and calcareous endoskeleton, and lack gill slits.

C. With Chordata

• Pharyngeal gill slits – the strongest chordate character; the gill bars are supported by skeletal rods, as in Amphioxus.
• Dorsal tubular nerve cord in the collar region (with a neurocoel), formed by infolding of dorsal ectoderm, like the chordate neural tube.
• Stomochord – once compared with the notochord (now regarded as a buccal diverticulum only).
• Deuterostome development and enterocoelous coelom (as in Amphioxus).
• Differences: no true notochord, no post-anal tail, no myotomes, no ventral heart; main nerve cord also ventral. So Balanoglossus is not a true chordate but a connecting link between non-chordates (Echinodermata) and chordates.

Likely exam questions

1. (Long, 8 marks) Describe the external features and digestive system of Balanoglossus with labelled diagrams.
2. (Long, 8 marks) Discuss the affinities of Balanoglossus with Annelida, Echinodermata and Chordata.
3. (Short, 4 marks) Give the general characters of Hemichordata.
4. (Short, 4 marks) Describe the Tornaria larva with a labelled sketch.
5. (Short, 4 marks) Write a note on the phylogeny (systematic position) of Hemichordata.