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!
- body has two tagmata: trunk and very reduced abdomen
- typically have 1 pair chelicerae (3-segmented), 1 pair ovigers, 4 pair walking legs, 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)
- body has two tagmata: prosoma and opisthosoma
- have 1 pair chelicerae (3-segmented), 5 pairs walking legs (first 4 with claws), abdominal book gills, and a prominent tail spine
- only living chelicerate with well-developed compound eyes

c) Cl. ARACHNIDA (spiders, mites and ticks, scorpions, etc.) ~80,000 spp. in 11 orders; hugely successful terrestrial group; some marine and freshwater

- generally inspire more fear in humans than any other animal group
- body has 1 - 3 tagmata (depends on 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 and sensory (spiders) or form large pincers (scorpions)
- 4 pair walking legs (abdomen may have other limbs), 3-5 pr simple eyes

- Or Araneae (spiders); ~40,000 terrestrial spp

- liquify prey with digestive enzymes injected via hollow chelicerae; imbibe fluids with a sucking pharynx and stomach
- have the impressive ability to spin silk from the abdomen
- some have quite elaborate social and courtship behaviors

- Or Acari (mites and ticks) approx. 40,000 described spp., but >500,000 spp? are everywhere! (marine, terrestrial, aquatic,aerial, subterranean, deep sea, hot springs)

- most (except ticks) are very small (< 1mm)
- tagmatization weak or absent (2 tagmata in free-living sp. only)
- predators, detritivores, scavengers, herbivores and parasites; all ingest liquids (from host, or dissolved by saliva)