CRABS LOBSTERS, SHRIMPS, ETC — BARNACLES 301 



characterised by the length and stoutness of the feelers or antennae, and the small 

 size of the claws. In the lobsters, on the other hand, of which Adacua gammarus 

 is the familiar representative, the antennae are shorter and more slender, and the 

 claws of the first pair of limbs are much larger and more powerful. The Norwegian 

 lobster {Nephrops norvegicus) has the large claws longer, more slender, and 

 covered with tubercles. Prawns (Pakemon) differ from lobsters in having the first 

 pair of limbs no larger than the three hinder pairs, while it is the second pair 

 which is the largest, although, like the latter, the first pair is armed with pincers. 

 In the brown shrimps of the genus Crangon the last joint of the first pair of limbs 

 is capable of being folded back on the next joint, and there is no long spine on 

 the head extending beyond the middle pair of antennae. 



To another group of crustaceans belong the mantis-shrimps, so called on 

 account of the resemblance of their enlarged pair of limbs to those of the mantises 

 or praying-insects. The common Sqtt,illa mantis of the Mediterranean, occasionally 

 met with in British waters, measures about 7 inches, and in some countries is used 



o*..^^,;\0r 



MAXTIS-SHRIMP. 



as food. As a well-known representative of another group, the rattle-spider 

 (Idotea tricuspidata), the sheep-worm of the Baltic fishermen, may be briefly 

 mentioned. This species is specially remarkable on account of the variability of 

 its markings, and the changes of colour it exhibits within extremely restricted 

 areas. Sometimes it is pale yellow, and at others pale brown, green, or red, while 

 in other cases it is spotted or striped longitudinally or transversely. Those individuals 

 which live near together, are, however, invariably alike, and in colour and markings 

 are perfectly adapted to their environment. On green seaweeds they are green, 

 while if the seaweeds are brown or red the rattle-spiders are coloured to match, 

 as they also are when living upon decaying vegetable matter. 



Omitting mention of the smaller forms of crustaceans, a short space 



BcirTici cIgs 



may be devoted to barnacles, which in the adult state are so unlike 

 ordinary crustaceans that few persons will believe that they belong to the same 

 class. When, however, the valves are open, the delicate organs from which they 

 derive their name of cirrhipeds will be seen to correspond to the limbs of crustaceans, 

 and in the young state they are free-swimming. They attach themselves head 

 downwards to rocks, piles, ships' bottoms, seaweeds, or the shells or bodies of 



