MALACOSTRACA. 



293 



those of the female are weak, except the last, which in both sexes 

 form paddles. They are marine forms, e.g. Mysis (without gills 

 on the thoracic legs, Fig. 142), Lophogaster, and Enphaiisia 

 (with gills on the thoracic legs). The last-named starts in life 

 as a Nauplius. As an adult it has luminous organs on the eye- 

 stalks, thoracic legs, and abdominal segments. 



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

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

 appendages, the 

 first three pairs 

 are maxillipedes, 

 the five other 

 pairs are jointed 

 walking legs 

 (whence the term 

 Decapod). 



Sub-order I. Mac- 

 rura. Abdomen 

 long. Hoinants 

 (lobster) ; Acph- 

 rops (Norway 

 lobster, sea cray- 

 fish) ; Astacits 

 (fresh-water cray- 

 fish) ; Palimirus 

 (rock lobster), 

 whose larva was 

 long known as 

 the glass - crab 

 (Phy llo so in a) ; 

 Pencils, a shrimp 

 which passes 

 through Xaup- 

 lius, Zorea, and 

 Mysis stages ; 

 Lucifer and Ser- 

 gestes are also 

 hatched at a 

 stage antecedent 

 to the Zooea ; 

 Crangon vuJgaris 

 (the British 

 shrimp) ; Pahc- 

 mon, Pandalus, 

 Hippo I y t e 

 (prawns) ; Galathea 



abo{ 



FIG. 140. Hermit-crab withdrawn from its 

 shell. The anterior appendages are broken 

 off. 



hd., Head ; th., thorax ; abd.. abdomen. 



(with the abdomen bent forwards) ; 

 Eiipagurus (hermit-crabs) ; Birgus tatro (the ter- 

 restrial robber or palm crab), in which the upper part of the 

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

 numerous vascular plaits. 



