l8o CRUSTACEA EUCARIDA DECAPODA CHAP. 



appendages characteristic of the Zoaea, namely, the first and 

 second antennae, mandibles, first and second maxillae, and two 

 pairs of biramous swimming maxillipedes and small third maxilli- 

 pedes. In the Metazoaea (B), as in the Anomura generally, 

 the third maxillipedes develop into biramous swimming organs, 

 a thing they never do in the Brachyura, and the rudiments of 

 the thoracic segments put in a first appearance. The abdominal 

 segments are already fully formed in the Zoaea stage, so that 

 here as in all other Zoaeas, the order of development from in front 

 backwards is disturbed by the precocious differentiation of the 

 abdominal segments. The next stage is the " Glaucothoe " (Fig. 

 123, C), which corresponds to the Megalopa of Brachyura (Fig. 

 125, p. 183). It differs from the adult Hermit-crab in the perfect 

 symmetry of its body, the segmented abdomen, and the presence 

 of five pairs of normal biramous pleopods. At this stage, which 

 lasts four or five days, it resembles closely a little Galatheid. 

 The asymmetry of the adult (Fig. 123, D) is now imposed upon 

 this larva by the migration of the liver, gonads, and green, glands 

 into the abdomen, and by the shifting of the posterior lobes of 

 the liver on to the left side of the intestine, which is displaced 

 dorsally and to the right. The gonad lies entirely on the left 

 side. The pleopods of the right side now degenerate, more 

 completely in the male than in the female, and this degeneration 

 is not completed until the little crab has found a shell and 

 lived in it for some time. If a shell is withheld from it, the 

 degeneration of the pleopods is much retarded, so that although 

 the Hermit-crab assumes its asymmetry without the stimulus of . 

 the spiral shell, yet this stimulus is necessary for the normal 

 completion of the later stages. 



Fam. 1. Pylochelidae. The abdomen is macrurous and 

 symmetrical, with all the limbs present. Pyloclieles (Fig. 118, 

 P . 173). 



Fam. 2. Pagnridae. The abdomen is asymmetrical, with 

 some of the limbs lost. The antennal scale is well developed, 

 and the flagella of the first antennae end in a filament. 



Sub-Fam. 1. Eupagurinae. The third maxillipedes are wide 

 apart at the base, and the right chelipedes are much larger than 

 the left. Parapagurus from deep-sea, Eupagurus from temperate, 

 especially north temperate seas. Pylopagurus. 



Sub-Fam. 2. Pagurinae. The third maxillipedes are approxi- 



