MALACOSTRACA, 273 



Order 4. Decapoda.— The shield is large and firm, and is fixed to 

 • the dorsal surface of all the thoracic segments. Of the thoiacic 

 appendages, the first three pairs are maxillipedes, the five other 

 pairs are jointed walking legs (whence the term Decapod). 

 Sub-order I. Macrura. — Abdomen long. Homarns (lobster) ; 

 Nephrops (Norway lobster, sea crayfish) ; Astacus (fresh- 

 water crayfish) ; Palinurus (rock lobster), whose larva was 

 long known as the glass-crab (Phyllosoma) ; Pencrus, a shrimp 

 which passes through Nauplius, Zosea, and Mysis stages ; 

 Lucifer and Sergestes are also hatched at a stage antecedent 

 totheZogea ; Crangon vulgaris (the British shrimp) ; Paltzmoii; 

 Pandalus, Hippolyte (prawns) ; Galathea (with the abdomen 

 bent inwards) ; Pagurus, Eupagurus (hermit crabs) ; Birgus 

 latro (the terrestrial robber or palm crab), in which the tipper 



Fig. 117. — Schizopod [Mysis jlexuosa), from side, 



b., Brood-pouch borne on posterior thoracic limbs ; o., otocyst 

 in tail. Note eight pairs of similar biramose thoracic feet. 

 The last two thoracic segments are not covered by the 

 shield. 



part of the gill-cavity is shut off to form a "lung," the walls 

 having numerous vascular plaits. 



Sub-order 2. Brachyura. — Abdomen short, and bent under the 

 thorax. It is narrow in the male, and does not usually bear 

 more than two pairs of appendages ; it is broader in the 

 female, and bears four paired appendages. The ventral 

 ganglia have fused into an oval mass. Cancer (edible crab) ; 

 Carcinus mcenas (shore crab) ; Portunus (swimming crab) ; 

 Dromia (often covered by a sponge) ; Pinnotheres (living 

 inside bivalves) ; Telphusa (a fresh-water crab) ; Gecarcinus 

 (land-crabs, only visiting the sea at the breeding season). 



History. — -Fossil Crustaceans are found in Cambrian strata, but the 

 highest forms (Decapoda) were not firmly established till the Tertiary 

 period. Some of the genera, e.g. the Branchiopod Estheria, living from 

 Devonian ages till now, are remarkably persistent and successful. How 

 the class arose we do not know ; it is probable that types like Nebalia 

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