DECAPODA 415 



Of the various examples of the order which are mentioned below, 

 all except Leucifei\ Birgus, and Gecarcinus occur in British waters. 



The most aberrant member of the Decapoda is the minute, pelagic 

 Leucifer (Fig. 273 D), which has a very slender, macrurous body with 

 an extremely elongate head, long eyestalks, no limbs on the last two 

 thoracic somites, no chelae, and no gills. Like the normally built 

 prawn Penaeus and the rest of the group (Penaeidea) to which both 

 belong, Leucifer starts life as a Nauplius. 



Leander, the common prawn, one of the Caridea, is macrurous like 

 the crayfish, but built for swimming rather than walking, with 

 phyllobranchiae, and with chelae only on the first two pairs of legs. 



Crangon, the shrimp, is related to Leander but has a broader and 

 flatter body, a very small rostrum, and the first leg subchelate. 



Nephrops, the Norway lobster, one of the Astacura, differs from 

 the crayfish in minor points, among others in having the podobranchs 

 free from the mastigobranchs. 



Homarus, the lobster, differs from Nephrops in size, form of 

 chelae, etc. 



Palinurus, the crawfish or spiny lobster, one of the Palinura, differs 

 from the crayfishes and lobsters in having a small spine in place of 

 the rostrum, no antennal scale (exopodite), and no chela on any leg. 

 It has a peculiar broad, thin schizopod larva, the Phyllosoma 

 (Fig. 266 C). 



Eupagurus (Fig. 292), the hermit crab, one of the Anomura, 

 lives in the empty shells of gastropod molluscs. It has a large, soft 

 abdomen, containing the liver and gonads, twisted to fit into the 

 shell, and without appendages on the right side, save for the uropods, 

 of which both pairs are present, roughened, and serve to hold on the 

 shell. The first three pairs of legs are as in a crab, the last two small 

 and chelate. 



Birgus, the robber crab, is a hermit crab which has grown too large 

 to use the shells of molluscs, and has accordingly re-developed ab- 

 dominal terga. It lives on land in the Indopacific region, and is 

 adapted to aerial respiration by the presence of vascular tufts on the 

 lining of the gill chambers. Its Zoaeae are marine. 



Lithodes, the stone crab (Fig. 293), is by origin a hermit crab, but 

 has lost the habit of living in shells and so thoroughly taken on the 

 build of the true crabs that only some asymmetry of the abdomen and 

 a few other minor points of structure betray its ancestry, even the 

 uropods being absent. 



Galathea, the plated lobster, another of the Anomura, is lobster- 

 like but has the abdomen bent under the thorax, and the last leg small 

 and slender and folded into the gill chamber. 



Porcella?ia, the china crab, related to Galathea^ has a form of body 



