π
HEMICHORDATA
β’ Habitat. whats the habitat: Exclusively marine
β’ Habit: Solitary or Colonial.
β’ Body Structure:
β Type:
- Mostly tubiculous
- Soft and fragile
- Vermiform
β’ General Features:
β Symmetry: Bilaterally symmetrical
β Segmentation: Unsegmented
β GermLayers: Triploblastic
β Coelom:
β Type: Eucoelomate (Enterocoelous)
β Divisions:
β Protocoel: Proboscis
β Mesocoel: Collar
β Metacoel: Trunk
β’ Body Wall:
β Epidermis:
β Type: Single layered
β Glands: Mucous glands present
β Dermis: Absent
β’ Digestion:
β AlimentaryCanal:
- Complete
- Straight or U-shaped
β BuccalDiverticulum: Foregut gives out a hollow buccal diverticulum into proboscis, once considered a notochord
β’ Gill Slits:
β Location: Dorso-lateral pharyngeal region
β Pairs: 1 to several pairs
β Feeding: Ciliary filter feeders
β Note: Shared character with Chordata
β’ Circulatory System:
β Type: Simple and open
β Heart: Dorsal heart
β Vessels:
- One dorsal longitudinal vessel
- One ventral longitudinal vessel
β’ Excretion:
β Organ: Proboscis gland or Glomerulus
β Mechanism: Connected to blood vessels for excretion
β’ Nervous System:
β Type: Primitive
β Components:
- Subepineural nerve plexus
- Dorsal collar nerve cord (hollow)
β’ Reproduction:
β Mode: Mainly sexual
β Sexes: Usually separate
β Gonads: One to several pairs
β Fertilization: External
β Development:
β Type: Direct or indirect
β Larval forms:
- Tornaria larva
β’ Notochord:
β Presence: Only in anterior part of proboscis
β Type: Stomochord (not homologous to chordate notochord)
β Description: Flexible hollow tube from embryonic gut roof extending into the proboscis and connected to oral cavity
β’ Classification: Four Classes among which Balanoglossus belong to class Enteropneusta
β’ Habitat:
β whats the habitat: Exclusively marine this
π
BALANOGLOSSUS (ACORN WORM OR TONGUE WORM)
β’ Habitat:
β Type: Marine
β Distribution: World-wide
β Location: Bottom of shallow water; some in deeper water
β’ Habit and Body Shape:
β Nature: Tubiculous, burrowing
β Form: Worm-like with soft, elongated, cylindrical body
β SizeRange: 10 cm to 2.5 m
β’ General Features:
β Symmetry: Bilaterally symmetrical
β’ Body:
β Divisions:
- Proboscis (Protosome)
- Collar (Mesosome)
- Trunk (Metasome)
β Proboscis:
β Description: Anterior, conical part tapering anteriorly
β Proboscis Stalk:
β Position: Posterior extension, continuous with inner dorsal collar wall
β Preoral Ciliary Organ:
β Location: Below stalk base
β Structure: U-shaped ciliated depression
β Function: Chemoreceptor
β Coelom:
β Name: Protocoel
β Opening: Mid-dorsal proboscis pore near base
β Collar:
β Description: Middle, short, thick belt-like part behind proboscis
β SurfaceFeatures: Circular grooves or elevations
β Collarette:
β Structure: Funnel-like anterior collar part surrounding proboscis stalk
β Mouth:
β Position: Ventral, below proboscis stalk inside collarette
β Coelom:
β Name: Mesocoel / Collar coelom
β Structure: May be divided by dorsal and ventral mesenteries
β Openings: Opens into first pair of brachial sacs via collar pores
β Trunk:
β Description: Posterior and largest part of body
β Features:
- Flattened with superficial annulations
- Mid-dorsal and mid-ventral longitudinal ridges
β Regions:
β Branchiogenital Region:
β GillPores:
β Description: Longitudinal rows on each side of mid-dorsal ridge
β Structure: Mounted on ridge-like elevations
β GenitalWings:
β Description: Thin, leaf-like extensions on sides
β Contents: Gonads
β Openings: Gonopores (microscopic)
β Note: Usually folded over to conceal gill-pores
β Hepatic Region:
β Description: Middle region with numerous paired transverse folds
β Structures: Hepatic caeca
β Color: Dark brownish or greenish
β Post-Hepatic Region:
β Description: Posterior tapering region
β Feature: Bears terminal anus
β’ Circulation:
β HeartLocation: Located in proboscis
β RespiratoryPigment: Vanadium
β’ Larval forms:
- Tornaria (larva of Balanoglossus)