2. Non-vertebrate Chordates II – Vocabulary Review

Lecture Learning Outcomes

• Subphylum Cephalochordata (lancelets)

  • Recall and describe the feeding structures of Branchiostoma (amphioxus).

  • Describe the mode of respiration in cephalochordates.

• Subphylum Urochordata (tunicates)

  • Compare cephalochordate respiration with ascidiacean respiration.

  • Recall the three groups (orders) of thaliaceans.

  • Compare the mobility of larvaceans, thaliaceans, ascidiaceans and cephalochordates.

Phylogenetic Context & Taxonomy

• Kingdom Animalia
  • Phylum Chordata
  • Core chordate features (larval or adult): dorsal hollow nerve cord, notochord, pharyngeal slits, post-anal tail, endostyle/thyroid homolog.

• Subphyla covered in this lecture
  • Cephalochordata – Class Leptocardii (lancelets)
  • Urochordata / Tunicata
  – Class Ascidiacea (sea squirts)
  – Class Thaliacea (pelagic tunicates: pyrosomas, salps, doliolids)
  – Class Larvacea (larvaceans)

Subphylum Cephalochordata (Lancelets)

General Features

• Genera Branchiostoma & Asymmetron; colloquial “amphioxus” = “sharp at both ends”.

• Size 2\text{–}8\,\text{cm}; worldwide, shallow marine sand.

• Rapid sand-burrowers; only anterior oral hood protrudes for filter feeding.

• Harvested as human food in parts of Asia.

• Spend life sitting in shells or land

Body Plan & Segmentation

• Trunk-dominated, minimal regional differentiation.

• Bilaterally symmetrical; metameric repetition (Structure repeated serially along body).

• V-shaped myomeres/myotomes separated by myosepta → efficient fish-like locomotion (limited but present).

• Gonads segmentally arranged.

• Notochord extends to extreme anterior (hence Cephalo-chordata); stiffens body for burrowing & swimming.

• Unique among chordates: possesses all five core features yet lacks a true brain & vertebral column.

External Feeding Structures (exam focus)

• Buccal cirri – straining guards around mouth.

• Wheel organ – ciliated ridges that draw water inward, has tentacles on it and gets water coming in.

• Hatschek’s pit/groove – mucus secretion; possible endocrine homology (adenohypophysis).

• Velum & velar tentacles – regulate entry into pharynx, Further sorting .

• Endostyle – ventral, mucus-producing, transports food.

• Pharyngeal bars (primary septa, secondary tongue bars) with cilia - traps the food then to pass the bars along body into the digestive system through oesophagous.

• Epibranchial groove – dorsal tract that carries food posteriorly toward gut.

• Oral opening which has structures helping it feed

Internal Anatomy

• Pharynx perforated by \sim 180 slits; opens laterally into the atrium – the main body cavity.
  • Coelom extremely reduced (confined to gonads, pericardial regions, etc.).

• Midgut caecum (blind pouch) on right side – homologous precursor to vertebrate liver & pancreas.

• Gut path: mouth → pharynx → oesophagus → caecum (dorsal limb) → midgut → ileocolic ring → hindgut → anus (separate from atriopore).

• Food transported to oesophagus (dorsal) and goes into the blind ended ceacum. Travels along the dorsal side of the caecum and returns on the ventral side. Caecum may be a precursor to the liver and the pancreas

• Waste materials exits via anus

Digestion & Excretion

• Food entangled in mucus sheet; cilia roll sheet into cord → epibranchial groove → gut.

• Waste passes via anus posterior to atriopore.

• No kidney: filtration by cyrtopodocytes associated with glomeruli; filtrate enters atrium.

Respiration

• Body small; no dedicated gills on pharyngeal bars – gas exchange primarily by diffusion:
  • Across thin pharyngeal epithelium.
  • Directly across vascularised atrial wall; axial muscles also satisfy O$_2$ demands cutaneously.

• Water route: mouth → pharynx → atrium → atriopore (ventrally placed); anus is separate.

• As small no need for specialised respitory system, low metabolic rate, don’t have separate gills.

Circulatory System

• Layout analogous to vertebrates but no heart.
  • Serially beating vessels (“bulbilli”) propel blood.
  • Flow direction: ventral aortae (anterior) → pharyngeal bars → paired dorsal aortae → midline dorsal aorta (posterior).
  • Colourless plasma; no respiratory pigments.
  • True through-flow, unlike the ebb-and-flow of tunicates.

Nervous & Sensory Systems

• Anterior thickening of nerve cord = cerebral vesicle (two regions vs. three in vertebrates).

• Unusual wiring: muscle fibres send projections to nerve cord (inverse of vertebrate innervation).

• Receptors
  • Ocelli (simple photoreceptors) along nerve cord- light sensitive.
  • Chemoreceptors on buccal cirri, velum & skin.
  • Tactile mechanoreceptors in epidermis (skin).

Reproduction & Development

• Separate sexes; external fertilisation in water column.

• Planktonic larva lasts 75\text{–}>200\,\text{days}, exhibits left-right asymmetry.

• Atrium absent in larva; forms at metamorphosis when metapleural folds grow ventrally & fuse.

• Food-gathering organs and full pharynx develop post-metamorphosis.

• Free swimming

Subphylum Urochordata (Tunicates)

Shared Adult Features

• Body covered by tunic / test of tunicin (cellulose-like) + connective-tissue-like matrix.

• Two siphons: incurrent (branchial/oral) & excurrent (atrial).

• Atrium spacious; coelom vestigial.

• Heart present but simple; periodically reverses beat direction (ebb-and-flow circulation).

Class Ascidiacea (Sea Squirts)

Adult Morphology & Lifestyle

• Sessile, sac-like; attach to hard substrates in shallow seas worldwide.

• Only two chordate traits retained: endostyle & perforated pharynx (a net like bag).

• Mantle muscles can contract suddenly → expel water ("squirting").

• Tube coming in, tube going out with pharynx in middle

• Outside of body covered by tunic/test that has two openings - inhalant and exhaling siophons

Larval (Tadpole) Stage

• Free-swimming, non-feeding; trunk + muscular tail with notochord & dorsal nerve cord.

• Two ganglia: cerebral (sensory/motor; light-sensitive ocellus) & visceral.

• Water flow: mouth → few pharyngeal slits → atrium → atriopore.

• As large we see all the features

Metamorphosis

• 3 Adhesive papillae secrete mucus to attach head-first to substrate.

• Tail, notochord, much of nerve cord resorbed for nutrients.

• Larval mouth → adult incurrent siphon; atriopore → excurrent siphon.

• Pharynx enlarges; gut & gonads reposition; cerebral ganglion lost; visceral ganglion persists below new siphons.

• Attaches to substrate by three adhesive papillae that secretes mucus

Feeding Mechanism (Filter Feeding)

• Cilia on pharyngeal stigmata draw water.

• Ventral endostyle secretes continuous mucus sheet trapping plankton.

• Mucus cord conveyed dorsally via dorsal lamina → oesophagus → stomach → intestine.

• Digestion enzymatic; absorption in intestine.

• Plankton also drawn in with the water. A ciliated glandular groove along the floor of the pharynx, the endostyle, secretes mucus that traps food particles via dorsal laminate to the entrance of short oesphagus

Respiration

• Pharyngeal basket with numerous stigmata attached to test on one side; richly vascularised.
  • Mantle contractions & ciliary action maintain flow.

• Water exits atrium through atrial siphon; gas exchange on gill bars.

Circulation

• Heart = peristaltic tube; lies in pericardial (coelomic) remnant.

• Reverses pumping direction every few minutes → alternates haemolymph pathway.

Reproduction

• Hermaphroditic, protogynous (eggs produced before sperm).

• Gonoducts open near atrial siphon.

• Solitary forms: external fertilisation → tadpole larvae.

• Colonial forms:
  • Sexual larvae plus asexual propagation via budding (internal or from stolon) → extensive colonies.

Nervous System (Adult)

• Visceral ganglion at pharynx base; supplies siphons & body wall.

• Subneural gland beneath ganglion; function unknown (immunity? endocrine?).

Representative Diversity

• Solitary: Clavelina fusca, C. picta, "Venus fly-trap" (Megalodicopia hians) – can actively close oral opening to capture prey.

• Compound/colonial: e.g. Sidnyum elegans, Botryllus planus (individual zooids share common cloaca; visible star-like exhalant openings).

• Stalked & lobed growth forms common; larvae of Didemnum spp. shown.

Class Thaliacea (Pelagic Tunicates)

General Traits

• Free-living, open-ocean; likely evolved from adult ascidians.

• Maintain a permanent tunic; reduced stigmata; atrial opening posterior.

• Many exhibit alternation of generations (sexual ↔ asexual).

• Orders to know (Learning Outcome):

  1. Pyrosomida – colonial pyrosomes.

  2. Salpida – salps.

  3. Doliolida – doliolids.

Order Pyrosomida (Colonial)

• Cylindrical colonies – hundreds/thousands of zooids lining a common cloaca; oral ends face inward.

• Water drawn in through individual branchial siphons, exits collective posterior aperture → slow jet propulsion.

• Photogenic cells; colonies bioluminesce when disturbed (anti-predator?).

• Giant colonies recorded: 10.2\,\text{m} long (>20\,\text{m} historically); individual zooids only 1.7\,\text{cm}.

Order Salpida (Salps)

• Solitary individuals can asexually bud long chains (aggregate phase).

• Alternate generations: solitary oozoid (sexual) ↔ aggregate blastozooids (asexual).

• Muscular body rings; water flow mouth→atrium→posterior opening generates modest jet thrust.

Order Doliolida

• Barrel-shaped, solitary.

• Propulsion by whole-body contraction.

• Life cycle: sexual tadpole-like larva → buds multiple zooid types, some sexual, some asexual.

Class Larvacea (Appendicularia)

Key Points

• Tiny (body \approx 8\,\text{mm}), deep-sea, paedomorphic – retain tadpole morphology throughout life.

• Tail with persistent notochord; trunk houses organs.

• Secrete a gelatinous mucous “house” with inflow & outflow filters; discarded & replaced frequently (minutes–hours).

• Tail beating drives water through house → fine mesh retains pico- & nanoplankton; important carbon “sinkers” in ocean.

• Always solitary & free-swimming; example genus Oikopleura.

• Hypothesised to have evolved via neoteny from ascidian ancestor.

• Filter feeders

Comparative Mobility (Learning Outcome 5)

• Ascidiacean adults – sessile; mobility restricted to siphon opening/closing & body contraction.

• Cephalochordates – limited active swimming/burrowing via segmented myomeres.

• Thaliaceans – pelagic drift; locomotion via whole-body contractions or jet propulsion; colony movement (Pyrosoma) or chain swimming (Salps).

• Larvaceans – continuously free-swimming; tail beats for both feeding and locomotion.

Comparative Respiration (Learning Outcome 3)

• Cephalochordates
  • Rely primarily on surface diffusion across thin pharyngeal bars & atrial wall.
  • No specialised gill lamellae; atriopore distinct from anus.

• Ascidiaceans
  • Possess true gill bars/stigmata with rich vascularisation; mantle pumps aid flow.
  • Gas exchange largely within pharyngeal basket; water exits atrial siphon; circulatory pattern ebb-and-flow.

Ethical, Ecological & Practical Notes

• Amphioxus as living model for basal chordate condition; informs vertebrate origins but collected for food/lab studies → potential overharvest.

• Tunicates (esp. Didemnum spp.) can be invasive, smothering benthic communities & fouling aquaculture gear.

• Thaliacean blooms (salps, pyrosomes) influence carbon sequestration by producing fast-sinking fecal pellets/ discarded houses.

• Larvacean “houses” crucial to marine snow; MBARI studies reveal deep-sea carbon transport.

Numerical / Statistical References & Formulas

• Body length amphioxus L = 2\text{–}8\,\text{cm}.

• Larval planktonic duration t = 75\text{–}>200\,\text{days}.

• Giant Pyrosoma colony L_{colony} = 10.2\,\text{m} (recorded; historical >20\,\text{m}).

Connections to Broader Zoology

• Endostyle ↔ vertebrate thyroid (iodine uptake, mucus secretion vs. hormone production).

• Segmental myomeres & hepatic portal system precursors to vertebrate muscular & circulatory organisation.

• Tunicate ebb-and-flow heart considered evolutionary stepping stone to unidirectional vertebrate circulation.

• Paedomorphosis (larvaceans) exemplifies heterochrony as macroevolutionary mechanism.