MARINE BIOLOGICAL STATION AT POUT ERIN. 97 



rock pool or on the sea-shore at low tide. These sea-weeds 

 are found to be attached to little hooked spines, and 

 observations have shown that the crab attaches the sea- 

 weeds to them itself, apparently with a view to its own 

 disguise. We have generally some specimens of the 

 spider-crab, Hyas araneus, showing this concealment in 

 the tanks at Port Erin. Some other kinds of crabs are so 

 shaped and coloured as to be very like the stones and other 

 objects amongst which they live. 



All the crabs, as well as lobsters and shrimps, as they 

 grow larger, periodically throw off or " cast " their hard 

 shells so as to permit of expansion. This process of 

 " ecdysis " may sometimes be seen taking place in an 

 aquarium, and it is so complete that not only is the outer 

 covering of the body shed, but every limb is drawn out of 

 its hard sheath, and the coverings of the eyes and the 

 delicate feelers and gills and even the cuticular lining of 

 the stomach are all cast off. For some time after this the 

 crab remains in a feeble and defenceless condition, but 

 swollen up with water, while its new shell is forming and 

 hardening. Such " soft " crabs generally hide, are rarely 

 caught, and are recognised as being unfit for eating. 



The life-history of the shore crab (Carcinus moenas) is 

 interesting, and figure XYIII. shows some of the more 

 important free stages. The developing eggs are carried 

 about as an orange or dark brown mass underneath the 

 abdomen, and when the young animals hatch out they are 

 called " Zoeas " (1) and are quite unlike the old crab. 

 They have a large jointed abdomen and several long 

 spines sticking out from the body. They swim freely on 

 the surface of the sea, and are frequently caught in the 

 tow-net in summer and autumn. After a time the Zoea 

 grows larger, casts its skin, and becomes the next stage, 

 or " Megalopa " (2), which is much more like a crab, but 



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