Hemichordata Flashcards
Key features?
bilateral symmetry
triploblastic body plan
asexual or sexual
benthic burrowers
how many species?
135
What are the classes?
Enteropneusta and Pterobranchia
where do they live?
marine - all ocean depths
where are they found?
E - buried in soft sediment
P - under/attached to rocks often in colonies
why are they interesting?
various means of reproduction, the classes differ when it comes to reproduction/anatomy, phylogenetically important for our understanding of deuterostome evolution
phylogeny of Pterobranchia
two subclasses; cephalodiscida and graptolithia
why is it difficult to tell when cephalodiscida separated?
Graptolitha are classified by their encrusted base, and there is debate over when encrusting originated - so could place them in different groups
what does Graptolithia further divide into?
rhabdopleuris and graptoloidea
how does enteropneusta divide?
harrimaniidae - earliest branch
splengelidae and ptychoderidae/torquarabridae derived later due to reproductive trait differences
why do we think they’re related to echinoderms?
suggests common ancestor with deuterostomes so share traits with chordata and echinoderms
whats the circulatory system?
partially open with blood flowing through tissue sinuses into sac of muscles which pulsates and acts as a heart - blood flow controlled by muscle contractions and a heart vesicle
whats the nervous system?
E - two nerve cords - dorasal and ventral in the collar that connect to nerve net in trunk
P - no nerve cord but nerve net in mesoderm at bottom of tentacles from which a collar ganglion forms
how do they eat?
e - lack tentacles
P - have a crown of 5 tentacles they use to catch food and pass it to the mouth
tentacles are transformed from the mesosome and are found in the collar
how does the digestive system work?
occurs within oesophagus stomach and intestine
why are they important?
tracing origins of deuterostomes
understanding relationships and developmental processes to further the knowledge in other related phyla
share morphological features and genetic patterns with chordates
regenerative abilites
understanding earths origins/drivers of cambrian - graptolites in fossil record since then
early development overview?
sexual and asexual reproduction
egg with radial holoblastic or sub-equal cleavage
blastula gastrulates by invagination
blastopore on posterior side - replaced by anus but closes before proper development of anus
development in pterobranchia?
eggs have high yolk content
larvae secrete coencium - attatched to dorsal side
mesoderm is rich in yolk - forms coelomic cavities - as develop yolk lost an physiological structures develop
development in rhadopleura (P)
short pelagic period - larvae settle and undergo metamorphosis
coelomic cavities form - determined by endoderm
outside features form until coecium breaks and a new colony is formed
development in cephalodiscus?
planula larva stage used for dispersal - some continue to develop without motile stage
development in enteropneusta?
direct vs indirect
harrimaniidae - direct
other subclasses - indirect
what happens in direct development?
more eggs produced - bigger w/ greater yolk content
how are ecto/endo and mesoderm formed?
endo and meso via lower vegetal tier
ecto - animal and upper vegetal tier
differences in development?
rate of development - direct faster as elongate earlier and more rapid specialisation of cells
timing of hatching - direct - fully developed from 5dpf
indirect - partially developed - 26hpf continue to develop for the next 50-65 days
