76 INVERTEBRATE ANIMALS. 



Section C. — Crabs with shields more or less semicircular in 

 front = Famille des Cyclometopes (Milne Edwards). 



Family PORTUNID^. Crabs with hinder tarsi 

 much dilated, for swimming. 



Group 182 — Genus CHARYBDIS (De Haan) and aUies. 

 In the large division of the Crustacea characterised by 

 having organs of vision set on long footstalks, Podoph- 

 thalmus vigil, in this group, an inhabitant of tropical 

 seas, affords the most striking example of pedunculated 

 eyes, the special advantage of which to the animal has 

 not been fully ascertained ; but it seems certain that 

 by such an arrangement the animal might compute 

 the distances of objects of prey very accurately by their 

 parallax, and might view them stereoscopically before 

 devouring them. 

 Estimated number of species : recent, 20 ; British, 0. 



^ Examples of Thalamita, Lissocarcinus, &c. 



Group 183.— Genus NEPTUNUS (De Haan) and allies. 

 Many of the species of this group are found on the 

 high seas far from land. Note the clean-cut lines of the 

 shield, the arms, and the swimming paddles in the 

 frigate-like Neptunus pelagiciis. The form of this ocean- 

 crab is a study in the art of combining prehensile 

 efficiency with strength and speed. 

 Estimated number of species : recent, 20 ; British, 0. 



^ Examples of Amjyhitrite, Oceanus crucifer, &c. 



Group 184.— Genus PORTUNUS (Fabricius) and allies. 

 The British Swimming-crabs are known to fishermen 

 by various local names, such as the common Shore-crab, 

 the Velvet-crab, the Fiddler-crab, the Nipper, &c. They 



