CHAPTER XVI 



THE ARTHROPODS 



AFTER the molluscs we come to the Arthropods, which are repre- 

 sented in the sea as typically by the Crustacea as they are by 

 insects in fresh water. The shore Crustacea include some very 

 interesting forms, and where fishes are found too difficult to 

 keep alive we must rely largely upon the Crustacea to give anima- 

 tion to the aquarium. We may begin with that invaluable 

 animal the common shore crab (Carcinus mcznas), which is not 

 only very common but is very hardy, and will thrive under the 

 most untoward conditions. Do not attempt to keep the large 

 forms so common both off rocky and sandy shores, but select 

 a few small specimens, and put them in flat dishes with stones 

 projecting out of the water. The animal is too familiar to need 

 description, but very little collecting experience will show that 

 the colour is very variable. Take a few young specimens from 

 the same pool, which will probably have roughly the same type 

 of coloration, and put them separately into pie-dishes with 

 differently coloured surroundings red weed in one, green in 

 another, dark pieces of shale in another, and so on. In the 

 course of a few days if young and small specimens have been 

 taken it will be found that the colour of each crab is in har- 

 mony with its surroundings. The specimens can then be changed, 

 and the experiment repeated ad lib. The crabs are actively car- 

 nivorous and should be well fed their tastes are very catholic. 

 Note in your specimens the short feelers or antennae, as com- 

 pared with the long ones of a lobster, the fact that the animal 

 has no power of swimming though it has a small tail, habitually 

 carried bent in under the body, and not capable of being 

 straightened like that of the lobster. As more subtle points 

 note the constant flicker of the appendages round the mouth ; 

 the object is to cause a current of water to flow over the gills, 



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