CLASSIFICATION OF CRUSTACEA 353 



Order i. Euphausiacea : — shrimp-like surface and deep-water forms, 

 with biramous thoracic limbs as in Mysids, e.g. Euphausia. 



Order 2. Decapoda : — with the three anterior thoracic limbs 

 turned forward as maxillipedes, with the other thoracic limbs almost 

 always uniramous. 



Sub-order Macrura. — Abdomen long. Homarus (lobster) ; Neph- 

 rops (Norway lobster, sea crayfish) ; Astacus (fresh-water crayfish) ; 

 Palinurus (rock lobster), whose larva was long known as the glass- 

 crab (Phyllosoma) ; Penceus, a shrimp which passes through Nauplius, 

 Zosa, and Mysis stages ; Lucifer and Sergestes are also hatched at a 

 stage antecedent to the Zoaea ; Crangon vulgaris (the British shrimp) ; 

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

 bent forwards) ; Pagurus, Etipagurus (hermit-crabs) ; Birgus latro (the 

 terrestrial robber or palm-crab), in which the upper part of the gill- 



FiG. 198. — Mysis flexuosa, from side. 



6., Brood-pouch borne on posterior thoracic limbs ; 0., otocyst 

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

 The last two thoracic segments are not covered by the 

 shield. 



cavity is shut off to form a "lung," the walls having numerous 

 vascular plaits. 



Sub-order 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)j 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 Anaspides 

 and Nehalia give us trustworthy hints as to the ancestors of the higher 



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