PANARTHROPODA: ONYCHOPHORA, TARDIGRADA & Arthropod origins
( Onychophora study images;
a) Why are arthropods so successful?
ii) segmented body + jointed limbs: both permit tremendous structural, developmental and functional versatility
iii) image-forming eyes: facilitate a huge variety of behaviors
b) What were the evolutionary origins of the Arthropoda?
- from a segmented annelid-like ancestor (Articulata hypothesis)
- from an unsegmented worm-like ancestor (Ecdysozoa hypothesis)
2) Ph. ONYCHOPHORA (velvet worms, 180 spp.) a small terrestrial phylum; living forms discovered in 1826; predators on insects
a) Has a worm-like body form similar to an errant polychaete
b) 'Missing link' shares many similarities with Annelida and Arthropoda:
- arthropod-like: cuticle must be molted; reduced coelom; spacious hemocoel (open circulatory system); foregut & hindgut lined with cuticle; respire with trachea/spiracle system (independent evolution); large brain; cleavage not spiral
c) unique trait: slime papillae; defensive- slime shoots out up to 50cm and hardens to ensnare potential predators
d) separate sexes; sperm transfer varies: most use spermatophores, some copulate; derived species are viviparous (embryo develops with placenta; live birth)
e) is the only, purely terrestrial animal phylum! However, marine examples are known from lower Cambrian (approx. 540 MYA; e.g., Hallucigenia)
3) Ph. TARDIGRADA (water bears, 1,200 spp.) mostly terrestrial and freshwater, some marine
a) very common but minute animals, most less than 1mm long; body form has four 'segments' with paired limbs and distinct 'claws' at the tips
b) share similarities with Annelida and Arthropoda:
- arthropod-like: cuticle must be molted; no circular muscle layer, discrete muscles attach to cuticle; reduced coelom; spacious hemocoel (open circulatory system, but no heart, likely due to small body size); fore- and hindgut lined with cuticle; form of sensory setae; cleavage not spiral
c) unique traits:
d) capable of cryptobiosis- can survive complete desiccation and extreme freezing multiple times; may extend life span to 70 years!
e) most have separate sexes; terrestrial species copulate, aquatic species exhibit a kind of necrophilia: the female lays eggs in her molt and the male mates with the molt!
Copyright (c) 2017 by A. Richard Palmer. All rights reserved.
(revised Dec. 19, 2016)