a) Cl. PYCNOGONIDA (sea spiders) ~600 marine spp
- typically small bodied (<5 mm) but deep sea and Antarctic species have leg spans > 70 cm!; predators mainly on hydroids
- very unusual body form: legs make up 50 - 80% of body mass!
- typically have 1 pair chelicerae (3-segmented), 1 pair ovigers, 4 pair walking legs, a reduced abdomen, and simple eyes
- unusual reproductive biology: males brood embryos on special legs (ovigers); only arthropod group with sole male parental care!
b) Cl. MEROSTOMATA (horseshoe crabs) only 4 living species
- 'living fossils' because they resemble 400 million-year old fossils
- large-bodied predators (to 40 cm) on small, infaunal invertebrates (food shredded or crushed by gnathobases of walking legs)
- have 1 pair chelicerae (3-segmented), 5 pairs walking legs (first 4 with claws), abdominal book gills, and a prominent tail spine
- only chelicerate with compound eyes (independently evolved)
c) Cl. ARACHNIDA (spiders, mites and ticks, scorpions, etc.) ~100,000 mostly terrestrial spp; some marine & freshwater spp
- generally inspire more fear in humans than any other animal group
- have 1 pair chelicerae; may be 3-segmented (small pincers- most groups) or 2-segmented (hollow fangs to inject poison- spiders)
- have 1 pair of pedipalps; may be leg-like & sensory (spiders) or form large pincers (scorpions)
- 4 pair walking legs (abdomen may have other limbs), simple eyes
- Or Araneae (spiders); ~37,000 terrestrial spp
- liquify prey with digestive enzymes & imbibe fluids with a sucking pharynx & stomach
- have the impressive ability to spin silk from the abdomen
- some have quite elaborate social & courtship behaviors
- Or Acari (mites & ticks) >500,000 spp? are everywhere! (marine, terrestrial, aquatic,aerial, subterranean, deep sea, hot springs)
- most (except ticks) are very small (< 1mm); lack tagamata
- predators, detritivores, scavengers, herbivores and parasites; all ingest liquids (from host, or dissolved by saliva)