78 INVERTEBRATE ANDIALS. 



The Gecarc'midc^, as the name implies, are Land- 

 crabs, inhabiting the warmer regions of all parts of the 

 world, except the interior of continents. Their gills are 

 adapted for respiration so long as they are kept moist, 

 a condition favoured by the nocturnal habits of the 

 animals. Once in the year, at the spawning season, 

 they are said to migrate in vast numbers together. 



The Pinnothe rides, or Pea-crabs, reside in the 

 bivalve shells of living Molluscs. 



The Mycteridce, for the most part, inhabit salt- 

 marshes. 

 Estimated number of species : recent, 50 ; British, 2. 



II Examples illustrating the four families. 



Section C. — Grabs 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 allies. 

 In the large division of the Crustacea characterised by 

 having organs of vision set on long footstalks, Podoph- 

 tlialmus 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. 



