Palaeozoic Arachnida of North America. 27 



SPECIAL PART 

 PHYLUM ARTHROPODA 



Invertebrates with a chitinous exoskeleton and usually segmented 

 appendages, with a prae-oral supraoesophageal brain composed of not 

 less than 2 pairs of ganglia, and a ventral chain of ganglia, which 

 shows different grades of concentration. Organs of respiration, when 

 present, in form of gill-books, lung-books, or tracheal tubes. Organs 

 of circulation, when present, never in the form of a closed system, 

 heart open at both ends, with side ostia. Excretory organs either in form 

 of modified nephridia or as malpygian tubes. Development direct or 

 indirect, in the latter case often with a complicated metamorphosis. 

 The phylum is composed of 10 classes. 



Key to the classes of the Phylum Arthropoda. 



1. Two pairs of prae-oral appendages (behind the eyes) 



Crustacea 

 + one pair of prae-oral appendages (behind the eyes) ... 2 



2. Legs biramous Trilobita 



+ legs uniramous 3 



3. Segmentation of the body homomeric 4 



+ segmentation of the body heteromeric 8 



4. Head with i pair of post-oral appendages (mandibles) and a 

 pair of papillae. Genital opening on penultimate segment. 



Legs with 2 claWs Protracheata 



+ head with at least 2 pairs of post-oral appendages. Legs 



with I, 2 or 3 claws 5 



5. Head with 2 pairs of post-oral appendages. Antennae 



short, not more than 7-jointed 6 



+ head with more than 2 post-oral appendages. Antennae 



long, with many joints 7 



6. Antennae simple. Tracheal spiracles at base of all legs. 

 Genital opening on third somite 



Diplopoda 



+ antennae with 4 basal joints, from the distal of which 



arise 2 i-jointed branches, the external with i bristle, 



the internal with 2 bristles. Respiratory organs absent. 



Genital openings at base of 2d pair of legs 



Pauropoda 



7. Three pairs of post-oral appendages in the head. First' 

 pair of legs modified as maxillipeds. Tracheal spiracles 

 numerous, never on the head. Genital opening on penulti- 

 mate segment Chilopoda 



