LECTURE 15 - Deuterostomata Flashcards
What are the main deuterostome innovations?
- Asteroidea
- Ophiuroidea
- Holothuroidea
- Echinoidea
- Crinoidea
- Enteropneusta
- Pterobranchia
- Cephalochordata
- Vertebrata
- Urochordata
What differentiates Deuterostomata from Protostomata?
- A major potentiating innovation of deuterostomes is GILL SLITS: a series of openings in the wall of the pharynx braced by bars of cartilage
- Deuterostomata is a much smaller and less diverse group than Protostomia but includes two radically different body plans in echinoderms and chordates
What do Deuterstomes use gill slits for?
Deuterostomes are filter feeders that pump seawater through a pharyngeal sieve –> gill slits
What is the fate of pharyngeal arches in human development?
- This first pharyngeal arch gives rise to oral jaws; the second becomes hyoid and jaw support
- In fishes, the posterior arches aid in brachial skeleton that supports the gills
- In tetrapods, the anterior arches transform into ear, tonsils, and thymus
ARCH 1
- Cartilaginous bar of mandibular arch
ARCH 2
- Cartilaginous template for developing jaw bones
HYOID
- Braces tongue and facilitates speech and swallowing
What is the Yunnanozoon?
- Deuterostome origins
- Overall body plan chordate-like with myomeres, a high dorsal fin and a series of filamentous gills, with dorsal and ventral rods that could be homologous with the notochord
(IN DEUTEROSTOMATA) What are Ambulacraria?
- Echinoderms and hemichordates
What are the Echinoderm themes?
- Echinoderms are large, marine, benthic, free-living animals
- Loss of ancestral deuterostome features such as bilateral symmetry, segmentation and gill slits
- Gain of pentaradial (5-fold) symmetry
- Gain of unique skeletal and locomotory features
- Extreme decentralization
Describe the function (but not phylogenetic) sequence of locomotory design
(IN ORDER FROM TOP TO BOTTOM)
- Radial animals with little power of directed locomotion
- Bilateria with directed locomotion and cephalization
- Ciliary locomotion in solid-bodied acoelomates
- Coelom acts as a hydrostatic skeleton to enable muscles to act antagonistically and permit forceful burrowing in worms
- Segmentation increase effectiveness of coelom
- Hollow limbs develop as external coelomic pockets
- Limbs enclosed in thick cuticle
- Jointed stiff limbs function as hinged levers in arthropods
BUT IN ECHINODERMS
[…]
- Coelomacts as a hydrostatic skeleton to enable muscles ot act antagonistically and permit forceful burrowing in worms
- Loss of segmentation, directed locomotion and cephalization
- Return to radial symmetry and hydrostatic locomotion (The echinoderm reversal)
What is the great echinoderm renunciation?
- Bilateral symmetry: LOST DURING DEVELOPMENT
- Segmentation: LOST
- Cephalization: LOST
- Brain and CNS: LOST
- Eyes: LOST (except for simple eyespots)
- Pharyngeal slits: LOST
- Vascular system: LOST (except for ‘lacunar tissue’)
- Excretory system: LOST (no nephridia)
- Reproductive system: SIMPLIFIED (no copulatory organ, accessory glands, gonoducts or seminal receptacle)
After losing all of those features, what did echinoderm get in exchange?
- A complex water vascular system
- Dermal ossicles
How are Echinoderms powered?
Echinoderms are powered by hydraulic devices called tube feet
Describe the water vascular system in Echinoderms.
- Water is taken in through pores in the MADREPORITE and then transported through a STONE CANAL to the RING CANAL around the mouth
- It then travels through RADIAL CANALS to each arm, where LATERAL CANALS extend to the ampulla and podium (tube foot)
- The AMPULLA works like an old-fashioned bicycle horn; when this bulb contracts, the TUBE FOOT protrudes
- As the ampulla relaxes, the tube foot shortens, and a suction disk at the base adheres to the surface
- Tube feet line AMBULACRAL GROOVES
How does locomotion by tube feet function?
- Water enters the system through the madreporite, is pressurized by the polian vesicles, and permits extension of a tube foot when the muscle bands in the ampulla contract
- A tube foot can be retracted by a longitudinal muscle, re-inflating the ampulla and causing the flat end of the tube foot to adhere by suction to a hard surface
- Alternative extension and retraction of the tube feet, under local control, can be used to crawl over the sea bed
What are spongy ossicles and mutable ligaments?
- The skeleton of echinoderms consists of calcareous ossicles with a spongy texture, the STEREOM, connected by collagenous ligaments
- The ligaments are under local neuronal control and can be locked or loosened to change body form
- The skeleton is produced by mesenchymal cells but functions primarily as an exoskeleton
In Echinoderms, what contains the mouth and gut?
- The central disk contains the mouth and gut
- Everything else is distributed, with arms being nearly independent
- There are no dedicated respiratory, circulatory or excretory systems
- There is no brain and thus no central decision-making capability
What is the phylogeny of extant echinoderms?
- Echinoidea
- Holothuroidea
- Asteroidea
- Ophiuroidea
- Crinoidea
- What are Crinoidea?
- Sea-lilies
- The only surviving class of Pelmatozoa, attached directly or indirectly through a stalk by the aboral surface
- True crinoids have been recorded from early mid-Cambrian, and were numerous by early Ordovician
- Flourished in early Palaeozoic, with 6000 fossil species described
- Now reduced to about 600 species
- Typically stalked, most living species unstalked
- Pennatulate arms extend from theca; originally 5 but 10 or 40-200 in most species
- Ciliary-mucus suspension feeders
How do Crinoids feed?
- Crinoid feed by filtering small particles of food from the sea water with their feather-like arms
- The tube feet are covered with a sticky mucus that traps any food that floats past
- Once they caught a particle of food, the tube feet can flick it into the ambulacral groove, where the cilia are able to propel the stream of mucus towards the mouth
Describe stalked crinoids.
- Stalked crinoids were abundant in the Palaeozoic but are now restricted to the deep sea
- They were largely responsible for the notion that the deep sea might be inhabited by ‘living fossils’ - the remnants of groups which had once flourished but are now much reduced in abundance and diversity
What are Asteroidea?
- Starfish
- Benthic animals with 5 (usually) ray-like extensions
- Mouth down; carnivores eating molluscs and other echinoderms
- Water vascular system and stereom
- External fertilization leading to a ciliary-band larva which feeds in the plankton before settling and metamorphosing as a benthic adult
- Numerous in all seas; about 2000 extant species
- Tube feet play a secondary role in feeding
- The quasi-independence of the arms leads to remarkable powers of regeneration
What are Ophiuroidea?
- Brittle stars
- Stellate body with arms sharply demarcated from disc; arms solid, closed ambulacral grooves, tube feet without suckers, not used for locomotion
- Anus lacking
- Active, using arms; deposit feeders and carnivores
Describe the arms or Ophiuroidea
- A brittle star’s arms are supported by ‘vertebral’ or ‘segmental’ ossicles, plates made from calcite
- These plates work together like ball and socket joints to give the arms flexibility
- The plates are moved by a type of connective tissue called mutable collagenous tissues which is controlled by the vascular system
What is autonomy in brittle stars?
- AUTONOMY is the shedding of a body part in response to an external stimulus without lasting injury
- A section of arm can be shed by a brittle star in about 1 second (hence the name)
- It is not caused by any violent muscle contraction but instead by the abrupt loss of tensile strength in the connective tissue of the intersegmental joint
What class do Basket Stars belong to?
- Ophiuroidea
What are Echinoidae?
- Sea urchins
- About 800 species, common in all seas
- Spines borne on endoskeletal test of closely fitted calcareous plates
- Benthic animals with mouth down, capturing animal or plant food; often powerful teeth
Describe the jaw apparatus in Echinoidea.
- ‘Aristolte’s Lantern’
- goes from mouth (surrounded by teeth) controlled by retractor muscles
- Up the oesophagus encased in a calcareous plate which is controlled by protractor muscles
How do Echinoidea defend themselves?
- Pedicellaria
- The surface of the test (their ‘skin’) is provided with numerous pedicellaria to trop and kill any settling animal
What is a tesselated armour?
- Perhaps 5-fold symmetry originally evolved because the pentagon is the simplest shape that can tesselate the plane without leaving a line of weakness
What are Holothuroidea?
- Sea cucumbers
- Leathery test with minute ossicles
- Lie on side or burrow; suspension or deposit-feeders
- About 1600 living species, rare as fossils
What is the body plan of Holoturian?
- Holothurians are more complicated than most echinoderms
- At the anterior end, the mouth is surrounded by a ring of tentacles which are usually retractable into the mouth
- These are modified tube feet and may be simple, branched or arborescent
- Body wall made of “ catch’ collegen
- There is a respiratory tree in the cloaca which is used for breathing
- There is also an opening circulatory system
(IN AMBULACRARIA) What are Hemichordata?
- Enteropneusta
- Pterobranchia
Describe the generalized Hemichordate.
CHORDATE CHARACTERS:
- Pharyngeal slits
- Dorsal nerve cord (partly tubular in some species)
NON-CHORDATE CHARACTERS:
- No notochord
- No postanal tail
- Planktonic larvae resembling those of echinoderms (ciliated and simple gut)
(IN HEMICHORDATA) What are Enteropneusta?
- The two main groups of enteropneusts are slow burrowers, using the proboscis to burrow through sediment, and may be either deposit feeders (which consume sediments and digest the organic matter, rather like earthworms in soil) or suspension feeders (which collect suspended particles from the water)
- Some of these worms may be very large; one species may reach a length of 2.5 metres, although most are much smaller
Describe the anatomy of Enteropneusts.
- Hemichordates are distinguished by a tripartite (threefold) division of the body
- At the forward end of the body is a preoral lobe, behind this a collar, and last comes a trunk
- The name ‘hemichordate’ means “half chordate,” and hemichordates share some (but not all) of the typical chordate characteristics
- There are branchial opening, or “gill slits,” that open into the pharynx; there is a rudimentary structure in the collar region, the stomochord, that somewhat resembles a notochord; and there is a dorsal nerve cord, in part tubular
- The circulatory system is open
(IN HEMICHORDATA) What are Pterobranchia?
- The alternative body plan of hemichordates is found in pterobranchs, a small group of marine organisms
- They have a crown of branched, ciliated filter-feeding tentacles resembling a lophophore
- Pterobranchs forms colonies in which the individuals are interconnected by stems, or STOLONS
- Individuals, or ZOOIDS, are often less than 1-mm long
- The proboscis is not elongated, as it is in acorn worms, but shield-shaped
- The second division of the body (the mesosome) bears a pair of branched tentacles that collect small food particles from the water
- There is only one branchial opening
- Most strikingly, almost all pterobranchs species create and live within a network of tubes, the COENECIUM
- These tubes are made of collagen
- Despite such different structure and way of life, similar larvae and a tripartite body plan unite the enteropneusts and pterobranchs
What are Graptolites?
- Graptolites are abundant as fossils in the lower Palaeozoic
- They disappeared completely from the fossil record in the Carboniferous
- They are now thought to have been pterobranch hemichordates
