XII CRUSTACEA 169 



A curious well-known fact about the hermit crabs is their 

 custom of living in partnership with certain other creatures ; 

 e.g. a beautiful sea Bristle-worm {Nereis fumta) is very 

 frequently to be found within the shell, whilst attached 

 externally there may be one or more special sea anemones 

 or a cluster of the little zoophyte Hydractinia. Though the 

 crab is carnivorous, all these associated forms live peaceably 

 together, the guests feeding off the discarded morsels of the 

 crab's meal. 



Sessile-eyed Crustacea 



Besides the Decapods already described, there are other 

 Crustacea which have more definitely segmented bodies, and 

 in which eight thoracic and usually six abdominal segments 

 bearing appendages can be recognised, but in which the 

 carapace is absent, or is represented merely by the fused 

 chitinous skin of the head and first thoracic segment. In 

 these forms the limbs are never branched, and the eyes 

 though compound are always sessile. 



To this group belong the fresh-water shrimps, the wood- 

 lice, and the water-slaters. 



Order 2 : Amphipoda 



The Fresh-water Shrimp (Oammarus) is a small 



Shrimps ^^ form, very plentiful indeed in rivers and ponds, 



where it acts as a scavenger, feeding on any dead 



animal or vegetable 



matter. 



The light brownish 

 body is about | an inch 

 long and is laterally 

 compressed. On the 

 first division of the body 

 can be seen two pairs 

 of long antennae and 



the first thoracic ap- Fig. 105.-The Fresh-water Shrimp 



pendages, the laws /. , ^ ,^ ,. 



f . ° ,'. . . ■' , (Natural size shown by the line above.) 



being hidden ; next 



come seven distinct thoracic segments, each bearing a pair of 



jointed legs with a plate-like gill attached to the base of each. 



