Protochordates Flashcards
How are the phylum Hemichordata described?
Linkage to chordates due to occurrence of notochord, dorsal hollow nerve chord, and gill slits
What 2 classes exist within the phylum Hemichordata?
Eneteropneusta- large solitary worms
Pterobranchia- colonies of small zooids
How are the class Enteropneusta described?
70 species
Body of proboscis, collar and trunk
Gill slits in branchiogenital region connect to pharynx
Outer surface of body ciliated with numerous gland cells to produce mucus
What is the life mode of the Enteropneusta?
Mostly burrowers in soft sediment in shallow water
Burrow with muscular movements of proboscis and cilia transport sediment along the outside of the body
How do the enteropneusta feed?
Deposit or suspension feeders
Particles trapped by mucus on proboscis, transported by cilia to mouth, rejected particles transported down trunk by cilia
How does the hollow dorsal nerve cord in the collar of the Enteropnuesta develop?
Formed by similar developmental process to formation of the neural tube in chordates
How are the class Pterobranchia described?
21 species
Colonial, tube dwelling, epibenthic
Zooids are small
Uncommon in shallow water- more often recorded from deep water and around antarctica
How are the class Pterobranchia structured?
Organ systems similar to enteropneusts
Oral shield- glandular and ciliated for movement within the tube
Short collar with arms bearing ciliated tentacles (feeding)
Trunk is sac-like with U-shaped gut and fleshy stalk connecting different zooids within the colony
How are the tubes in the class Pterobranchia secreted?
Colonies in branching tubular network
Tubes secreted by oral shield
What sub phylum exist in the phylum Chordata?
Urochordata
Cephalochordata
How are the phylum Chordata described?
No backbone but at some stage in life cycle they have the following distinguishing characteristics of chordates:
Notochord
Dorsal hollow nerve cord
Gill slits
Postanal tail
How are the sub phylum Urochordata described?
1300 species
Adults with complex exoskeletal tunic
Highly developed perforated pharynx
Notochord and nerve cord absent in adult but present in larval stage
What classes exist in the sub phylum Urochordata?
Ascidiacea (sea-squirts, sessile)
Thaliacea
Larvacea
How is the class Ascidiacea described?
Free swimming tadpole larva with notocord and neural tube
Adults sessile, solitary, or colonial
Body covered by a tough outer tunic
Most of body occupied by pharynx- feeding structure perforated by gill slits
Well developed blood system- periodic reversal of direction of flow
Accumulate vanadium in specialised blood cells-function linked to tunic protection
When are the chordate characteristics seen in the class Ascidiacea?
Chordate characteristics not evident in adult but can be seen in in larvae- free swimming tadpole larva with notochord and neural tube
How do the class Ascidiacea feed?
Highly efficient filter feeders
Mucus is produced by endostyle and carried over inner surface of pharyngeal basket by cilia
What does the mucus in the class Ascidiacea form?
Mucus forms a net like sheet
Mesh is very fine allowing it to sieve out particles as small as bacteria
How is the mucus sheet gathered in the class Ascidiacea?
Mucus sheet is carried round inside of pharyngeal basket gathered up by cilia on the opposite side of the basket to form a mucus cord for transport to the mouth
How are the gill slits in the class Ascidiacea structured?
The complexity of the gill slits in the pharyngeal basket varies in different species and is used as a taxonomic feature- increasing complexity creating greater surface area and enhanced filtration efficiency
What increases filtration efficiency in the class Ascidiacea?
There are often structures on the inner surface of the pharynx to create a space between the mucus sheet and pharyngeal basket for increased filtration efficiency
How is the tunic structured in the class Ascidiacea?
The tunic is tough and is composed of a type of cellulose (tunicin) arranged in a layered plywood-like structure
Although effectively outside the body wall the tunic may contain blood vessels
Tunic may contain spicules or incorporate sand grains for greater strength
It is not moulted and grows with the body
As well as protection the tunic provides a means of attachment to the substrate
How are the colonial Ascidiacea arranged?
Colonial ascidians show a range of levels of integration of the separate zooids
In simpler forms isolated zooids are connected by stolons
The distal end of the zooids may be separate but united at the base by a common tunic
How are the zooids contained in the in the class Ascidiacea?
The zooids may be entirely contained within a common tunic but retain independent siphon openings to the surrounding water
Where are most of the class Ascidiacea found?
Most species found in shallow coastal water but also represented in deep sea- often stalked in this sedimentary environment
Some unusual forms (deep sea) are predatory rather than filter feeders
How are the class Thaliacea described?
Planktonic and transparent
May be very abundant in tropical or semi-tropical waters
Very efficient filter feeders (similar process to ascidians)
Complex life cycle with multiple stages (at least 1 stage colonial)
What are the pyrosomes?
Class Thaliacea
Colony in the form of closed cylinder
Size from a few cm to >3m
Luminescent
How are the class Larvacea described?
70 species- planktonic
Adults mostly- 5mm in size- resemble ascidian tadpole larvae
Lack tunic- secrete mucilaginous house
What is the house in the class Larvacea?
house has channels and filters- feeding current set up by beating tail
It is repeatedly shed and rebuilt as filters clog
Very effective filter feeding system- feed primarily on nanoplankton inc bacteria
How are the sub-phyla Cephalochordata?
25 species
Were initially regarded as molluscs, later considered to be fish- important evolutionary intermediate between invertebrates and vertebrates
Translucent
How are the sub phylum Cephalochordata structured?
Have notochord- composed of modified muscle- functions in stiffening body
Hollow dorsal nerve cord
Postanal tail
Pharyngeal gill slits
Where do the sub phylum Cephalochordata?
Lives in coarse sand/gravel in tide-swept shallow water
Burrows with head at surface and filter feeds
How is the feeding current set up in the sub phylum Cephalochordata?
Feeding current set up by specialised ciliated structure in mouth
Filtration mechanism similar to that of urochrodates- pharyngeal mucus net
