Arthropods and Trilobites Flashcards
Arthropod features
Great diversity
Metamerically segmented
Evolutionary modification by groups of segments becoming specialised
Tagmatisation of segments = tagmata, head, thorax and abdomen
-cephalothorax = head & thorax
Specialised appendages on each segment
-on head may become antenna or mouth parts
-used for movement on abdomen or thorax
Hox genes
Discovered in drosophila
Regulatory genes control anterior-posterior axis of animals (found in cnidarians too)
Tell cells which segment their in n what they will do
Code for regulatory proteins
Conserved in animal kingdom - do similar things throughout
Differences in arthropod plan due to differential expression (like no. of antennae)
Arthropod cuticle
Exoskeleton
Accounts for lots of success and allowed movement to land
Secreted by epidermis
Consist of - Epicuticle (top) made of proteins and wax (waterproof and lightweight) can avoid dessication
Procuticle - from protein and chitin and forms glycoprotein further divided into
-exocuticle (is tanned = cross-linkages with proteins of glycoproteins, very hard)
-endocuticle - softer layer
Salts can be used to harden it further - if aquatic, doesn’t matter that its heavy
Arthropod features (movement, growth, exchange etc.)
Not easy to move now - no peristalsis
Muscles attached to cuticle - contract and operate appendages
-appendages have thinner cuticle so can bend
Haemocoel circulation - heart present but pumps into surrounding tissues of haemocoel
Special structure for mass exchange - cuticle not permeable
Growth by ecdysis
Getting rid of old cuticle
Epidermis secretes new cuticle by proenzymes in moulting fluid
New epicuticle starts forming behind
Chitinase and proteinase in M fluid activated and digests endocuticle (absorbs products)
New cuticle forms under epicuticle
old epicuticle remains as cannot be broken down is shed
Soft new cuticle has been formed, what next…
Animal takes up water through osmosis or puffs up with air (makes bigger)
Cuticle hardens - then loses water or air - can grow into new cuticle
Growth curves show levelling off which this then followed by a moult, continue growing after and levels off etc.
In moults, new features can form, larval forms from adult = instars
Risk of predation - crustaceans can form massive congregations to moult traduce risk
Cuticle problems
Size limited - if big, need big cuticle for it to be effective, which becomes heavier and harder to move
Marine E can support larger cuticle
Need to allow effective gaseous exchange still
Trilobites morphology
3 body regions - head, thorax and pygidium (tail)
Tail region has anus
Appendages associated with thorax are unspecialised
Biramous limbs = two branches, filamentous (gill) and walking part
Limb backs into gnathobase - grinding surface used for feeding
Trilobite sensory apparatus
Compound eyes - seen in arthropods commonly, made of ommatidia
-consists of lens, leading to pigment cell and nervous connections below (lots)
-wide field of view, but vision not too great
Pair of antennae and more specialisation in head appendages
Trilobite life history
Were around 270mys, very successful, from 500mya
Some predacious, some scavengers and some filter feeders
70% said to have bite marks of right hand side…
-bad vision on that side making them vulnerable?
-predators good at attacking there
-not entirely sue
Phylogeny
Chelicerates and myriapods are grouped - molecular evidence of similarities
Crustaceans and hexapods together
-share various features and molecular similarities
-share mouth parts etc. (both have mandibles)
-fossils from Soviet Union showed a fossilised insect with vestigial legs attached to normal legs, suggesting they were once biramous like the crustaceans (evidence they evolved from them)
-similar brain and visual system wiring of crustaceans and insects
