8 Ecdysozoa and the Arthropods

what are the characteristics of Arthropoda

chitin exoskeleton, segmented with paired limbs, group-specific patterns of tagmatisation, species-rich and very abundant, especially insects

what is tagmatisation

the evolutionary process by which the body of an organism becomes divided into specialised segments or regions known as tagmata, that perform different functions

what are Onychophora

the velvet worms

what are the characteristics of the velvet worms

soft body with lobe-like limbs, hunt other invertebrates in tropical forests in the southern hemisphere using glue, had a marine origin and were long seen as a possible link between annelids and arthropods

what are the characteristics of the tardigrades

very small, found in all aquatic habitats, body composed of head plus 4 limb-bearing segments, feed using a pointed, hollow stylet

what are the characteristics of the nematodes

thread-like worms, usually very small, morphologically similar to each other, no limbs, no segmentation, no circulatory or gas exchange structures, novel excretory system, only have longitudinal muscles, usually feed using a stylet, inhabit almost all environments, many are parasitic with complex lifecycles

why were arthropods long seen as related to annelid worms via something like an onychophoran worm

because of similar segmentation

what are the traits that ecdysozoa share

a cuticle made of chitin, and growth by moulting (ecdysis), no external cilia and loss of ciliated larval forms, coelomic spaces are largely replaced by a haemocoel

what is the phylum that shows the clearest evidence of coelomate ancestry

onychophora

where is the coelomic space retained in onychophora

around the heart (pericardium) (the rest of the body cavity is a haemocoel

what is the structure of a generalised arthropod segment

one pair of limbs, organs and muscles in the haemocoel, no peritoneum covers the organs, which are in direct contact with the blood, dorsal heart(s), ganglion of paired ventral nerve cord

how is excretion carried out in onychophorans

by segmentally repeated mesodermal nephridia, as in annelid worms

what is the excretory system in nematodes

part of the epidermis (ectodermal renette cells), very simple duct systems

what is the excretory system in insects

derived from the gut (endodermal Malpighian tubules)

what is chitin a polymer of

N-acetyl glucosamine (structurally similar to cellulose)

what is the chitin exoskeleton secreted by

the underlying epidermis, often with layers conferring different properties

what do protein fibres in the chitin matrix lead to

can add hardness (sclerotisation of jaws) or elasticity (joints, wings)

what can minerals in the chitin matrix add

hardness

which mineral is common in crustacean arthropods

CaCO3

what do metals in the chitin exoskeleton add

strength/stiffness without making the exoskeleton brittle

what do waxes on the cuticle exterior add

waterproofing (this is common in arthropods)

what could explain the persistent moulting in some arthropods

the need to replace damaged waxes

how long do silverfish live and how many times do they usually moult

can live up to 8 years and moult up to 70 times

what do waterproof cuticles help small Ecdysozoa to achieve

cryptobiosis

outline cryptobiosis

an ametabolic state entered in response to adverse environmental conditions such as desiccation, freezing, or oxygen deficiency, all metabolic procedures stop, preventing reproduction, development, and repair, cryptobiotic organisms can survive for decades, improving conditions stimulate reactivation