10 



Abdomen large in Homarus (lobster), Nephrops (Norway lobster), 

 and Astacus (crayfish); small in Cancer (crab). [Homarus 

 larvse set free at shrimp stage.] 



(c) Stomatopoda. — Short carapace, only covering part of 

 thoracic segments. 



2. Hbdhiophthalamata. — Eyes sessile. No carapace, e.g. Gammarus 

 (sand-hopper) ; Oniscus, wood-louse (land form). 



II. TEACHEATA. 



Characters. — Tracheata have the characters of Arthropoda and in addition 

 the following : — They breathe air by tubes (tracheai) or slight modifications of 

 air-tubes. Never more than one pair of antennce. Jaws greatly modified. 



STJE-UIVISIONS. 



1. Peototbacheata (Peripatidea). — Worm-like arthropods (especially 



with worm-like excretory organs), with appendages only slightly 

 differentiated, e.g. Peripatus. , 



2. Myeiapoda (Centipedes and Millipedes). — Terrestrial {i.e. not aerial) 



forms. The body segments numerous and nearly alike, and all 

 have a pair of appendages, e.g. Julus ; Scolopendra. 



3. Insecta (Hexapoda). — One pair of antennse. Compound eyes. Head 



distinct. Thorax composed of three segments, with three pairs of 

 legs, and mostly two pairs of wings. Abdomen without looomotory 

 appendages. Eespiration by tracheae. Usually a larval asexual form, 

 differing widely from the adult, e.g. Blatta (Periplaneta, cockroach), 

 which, however, has an incomplete metamorphosis, and front wings 

 only form wing covers. (For Sub-divisions see Genealogical Table.) 



4. Araohnida. — Air-breathing arthropods, usually with cephalo-thorax 



and abdomen. Antennae wanting, but six pairs of looomotory appen- 

 dages attached to cephalo-thorax. Abdomen without appendages. 

 Eespiration by tracheae or respiratory sacks. 



(a) Scorpionidea ( Arthrogastra ). — Abdomen segmented. 

 Breathe by respiratory sacks, i.e. respiratory lamella} of the skin 

 simk into depressions of the body, and containing air. e.g. Scorpio. 

 [ The respiratory sacks are often called lung-books.] 



(6) Araneina (Spiders).— Abdomen unsegmented. Breathe by 

 respiratory sacks. 



(c) Aearina (Mites). — Degraded forms. Abdomen unsegmented, 

 Eespiratory organs, when present, in the form of tracheae. 



LIMULUS (King Crab). Is a peculiar Arachnid-like Arthropod, the 

 chief peculiarity of which is, that respiration is carried on by respiratory 

 lamellcB, like those in Scorpion ; but they contain blood, and are called gill- 

 books. The hinder part of the body is fused into one mass, and the terminal 

 part drawn out into a spine. Larvte resemble the extinct trilobites. 



