Protochordates

Question 1

How are the phylum Hemichordata described?

Answer 1

Linkage to chordates due to occurrence of notochord, dorsal hollow nerve chord, and gill slits

Question 2

What 2 classes exist within the phylum Hemichordata?

Answer 2

Eneteropneusta- large solitary worms
Pterobranchia- colonies of small zooids

Question 3

How are the class Enteropneusta described?

Answer 3

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

Question 4

What is the life mode of the Enteropneusta?

Answer 4

Mostly burrowers in soft sediment in shallow water
Burrow with muscular movements of proboscis and cilia transport sediment along the outside of the body

Question 5

How do the enteropneusta feed?

Answer 5

Deposit or suspension feeders
Particles trapped by mucus on proboscis, transported by cilia to mouth, rejected particles transported down trunk by cilia

Question 6

How does the hollow dorsal nerve cord in the collar of the Enteropnuesta develop?

Answer 6

Formed by similar developmental process to formation of the neural tube in chordates

Question 7

How are the class Pterobranchia described?

Answer 7

21 species
Colonial, tube dwelling, epibenthic
Zooids are small
Uncommon in shallow water- more often recorded from deep water and around antarctica

Question 8

How are the class Pterobranchia structured?

Answer 8

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

Question 9

How are the tubes in the class Pterobranchia secreted?

Answer 9

Colonies in branching tubular network
Tubes secreted by oral shield

Question 10

What sub phylum exist in the phylum Chordata?

Answer 10

Urochordata
Cephalochordata

Question 11

How are the phylum Chordata described?

Answer 11

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

Question 12

How are the sub phylum Urochordata described?

Answer 12

1300 species
Adults with complex exoskeletal tunic
Highly developed perforated pharynx
Notochord and nerve cord absent in adult but present in larval stage

Question 13

What classes exist in the sub phylum Urochordata?

Answer 13

Ascidiacea (sea-squirts, sessile)
Thaliacea
Larvacea

Question 14

How is the class Ascidiacea described?

Answer 14

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

Question 15

When are the chordate characteristics seen in the class Ascidiacea?

Answer 15

Chordate characteristics not evident in adult but can be seen in in larvae- free swimming tadpole larva with notochord and neural tube

Question 16

How do the class Ascidiacea feed?

Answer 16

Highly efficient filter feeders
Mucus is produced by endostyle and carried over inner surface of pharyngeal basket by cilia

Question 17

What does the mucus in the class Ascidiacea form?

Answer 17

Mucus forms a net like sheet
Mesh is very fine allowing it to sieve out particles as small as bacteria

Question 18

How is the mucus sheet gathered in the class Ascidiacea?

Answer 18

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

Question 19

How are the gill slits in the class Ascidiacea structured?

Answer 19

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

Question 20

What increases filtration efficiency in the class Ascidiacea?

Answer 20

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

Question 21

How is the tunic structured in the class Ascidiacea?

Answer 21

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

Question 22

How are the colonial Ascidiacea arranged?

Answer 22

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

Question 23

How are the zooids contained in the in the class Ascidiacea?

Answer 23

The zooids may be entirely contained within a common tunic but retain independent siphon openings to the surrounding water

Question 25

Where are most of the class Ascidiacea found?

Answer 25

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

Question 26

How are the class Thaliacea described?

Answer 26

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)

Question 27

What are the pyrosomes?

Answer 27

Class Thaliacea
Colony in the form of closed cylinder
Size from a few cm to >3m
Luminescent

Question 28

How are the class Larvacea described?

Answer 28

70 species- planktonic
Adults mostly- 5mm in size- resemble ascidian tadpole larvae
Lack tunic- secrete mucilaginous house

Question 29

What is the house in the class Larvacea?

Answer 29

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

Question 30

How are the sub-phyla Cephalochordata?

Answer 30

25 species
Were initially regarded as molluscs, later considered to be fish- important evolutionary intermediate between invertebrates and vertebrates
Translucent

Question 31

How are the sub phylum Cephalochordata structured?

Answer 31

Have notochord- composed of modified muscle- functions in stiffening body
Hollow dorsal nerve cord
Postanal tail
Pharyngeal gill slits

Question 32

Where do the sub phylum Cephalochordata?

Answer 32

Lives in coarse sand/gravel in tide-swept shallow water
Burrows with head at surface and filter feeds

Question 33

How is the feeding current set up in the sub phylum Cephalochordata?

Answer 33

Feeding current set up by specialised ciliated structure in mouth
Filtration mechanism similar to that of urochrodates- pharyngeal mucus net