- no antennae, chelicerae as mouthparts, uniramous limbs, simple single-lensed eyes (exception: Xiphosura).
- ancestral structure of chelicerae is three segments with the last two forming a pincer (i.e. are chelate): spiders and many mites have chelicerae with only two segments.

b) Cl. PYCNOGONIDA (sea spiders) ~600 entirely marine spp

- from 1 mm to 70 cm in leg-span, mainly < 5 cm
- have chelicerae and one pair of sensory palps
- mostly predatory on hydroidss, but some feed on algae or detritus
- but, have many differences from chelicerates: opisthosoma so reduced as to be nonexistant, mouth at the end of a tubular proboscis, has an extra pair of legs in males called ovigers, have gonads in their legs, have multiple gonopores on their legs, and some species have 5 or 6 pairs of walking legs
- males brood embryos

c) Cl. MEROSTOMATA (extinct sea scorpions, horseshoe crabs)

• Or. Eurypterida (sea scorpions):
  
  - entirely extinct order of marine and freshwater predators
  - got quite big - up to 3 m, the largest (by mass) of any arthropod
  - had pair of compound eyes and probably one pair of ocelli as well

• Or. Xiphosura (horseshoe crabs):
  
  - only 4 extant species
  - 1 pair of chelicerae, 5 pairs of walking legs (first 4 with claws), abdominal book gills, and a prominent tail spine
  - like eurypterids have 1 pair of compound eyes and 1 pair of ocelli
  - extant spp. are scavengers and predators on small, benthic invertebrates (food shredded or crushed by gnathobases of walking legs)
  - can get quite large, up to 60 cm (including tail spine)

d) Cl. ARACHNIDA (spiders, mites & ticks, scorpions, etc.)

- ~100,000 mostly terrestrial spp; ~7,000 marine & freshwater spp
- spiders, scorpions, mites, daddy-longlegs, psuedoscorpions, etc.
- internalized respiratory systems (book lungs and/or tracheae) and no compound eyes in extant species
- almost entirely terrestrial, some mites freshwater or marine
- 6 pr. anterior appendages (chelicerae, palps, and 4 pr. walking legs)
- most orders composed entirely of fluid-feeding carnivores that externally digest their prey before sucking up the juices.

• Or SCORPIONES (scorpions)
  
  - approx. 1,300 extant spp. in the world
  - largest extant species is 21 cm; extinct ones got up to almost 1 m
  - abdomen is divided into a pre- and postabdomen, the latter with a venomous sting, and pedipalps are modified as pincers
  - all predatory

• Or ARANEAE (spiders)
  
  - about 37 000 named species
  - abdomen without external evidence of segmentation; very tight constriction between prosoma and opisthosoma
  - pedipalps modified for sensing rather than prey capture
  - silk glands exit through spinnerets; silk used for building retreats and egg sacs, or to capture prey
  - all predatory and all terrestrial except for one European species that builds a 'diving bell'

• Or ACARI/ACARINA (mites)
  
  - ~ 48,000 named spp, perhaps up to 1,000,000 in total
  - often ranked as a subclass of the Class Arachnida
  - have a tubular gnathosoma that bears chelicerae, palps attached to unsegmented idiosoma (i.e., 2 tagmata: gnathosoma & idiosoma)
  - typically very small, from 0.01 mm to 1 cm for some ticks
  - 6-legged larval stage
  - terrestrial, marine, FW
  - predators, detritivores, scavengers, herbivores and parasites
  - the most economically important group of arachnids