OMNIVOROUS MOLLUSCS, INSECTS, AND CRUSTACEANS 253 



CRUSTACEANS (Crustacea) 



Among scavenging forms like those included in the Higher 

 Crustacea (Malacostraca), though animal food, including carrion, 

 is the favourite diet, omnivorous species might well be expected 

 to exist, and this is actually the case. A good example is the 

 Common Crayfish {Astacus fluviatilis) 

 (fig. 464) of our rivers, which resembles 

 a small lobster in appearance and in 

 the most essential features of its struc- 

 ture. It feeds upon small frogs, fishes, 

 and the like that venture within its 

 reach, and does not disdain offal, which 

 is indeed the most effective bait em- 

 ployed in its capture. Invertebrates 

 such as worms and other small creatures 

 are also appreciated, and the diet also 

 includes a considerable variety of plants 

 and roots. During the warmer parts 

 of the year crayfishes commonly leave 

 the water in the evening, and search for 

 food along the river banks. They live 

 in any natural crannies which happen to be available, and also 

 burrow extensively in the river banks, often doing a considerable 

 amount of mischief In this way. During the winter season they 

 adopt a more passive mode of feeding than at other times, lurking 

 at the mouth of their dwellings in wait for such toothsome morsels 

 as may pass within their reach, and which are secured by their 

 powerful nippers. The long sensitive feelers are able to explore 

 a considerable area round the mouth of the burrow, and constitute, 

 so to speak, an intelligence department giving prompt notice of 

 the presence of edible objects. 



A curious omnivorous form is found in a freshwater Prawn 

 {Atya), which inhabits the West Indies, and is about the size 

 of an ordinary marine prawn. The limbs corresponding to the 

 nippers of a crayfish are comparatively small, but end in slender 

 pincers, and the first pair of legs closely resemble them. The 

 food consists of the organic matter abundantly present in the 

 mud where the prawn lives, and the pincers are used for scraping 



Fig. 464. — Common Crayfish [Asiaczis 

 Jluviatilis), reduced 



