ONYCHOPHORA, TARDIGRADA & Arthropod origins ( Onychophora study images;
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2) Ph. ONYCHOPHORA (velvet worms, 110 spp.) a small terrestrial phylum; living forms discovered in 1826; predators on insects
a) 'Missing link' shares many similarities with Annelida and Arthropoda:
- arthropod-like: cuticle must be molted; reduced coelom; spacious hemocoel (open circulatory system) with a muscular heart; fore- & hind gut lined with cuticle; respire with trachea/spiracle system (independent evolution); large brain; cleavage not spiral
b) unique trait: slime papillae; defensive- slime shoots out up to 50cm and hardens to ensnare potential predators
c) separate sexes; sperm transfer varies: most use spermatophores, some copulate; derived species are viviparous (embryo develops with placenta; live birth)
d) 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, 800 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 (but no heart, likely due to small size); fore- and hind-gut 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) 2012 by A. Richard Palmer. All rights reserved.
(revised Dec. 31, 2011)