THE ARTHROPODS 



FIG. i^. 

 tricuspidata. 



process by shovelling sand over its back by the help of the large 

 scales which occur at the base of the feelers. Prawns should be 

 studied especially from the point of view of 

 their methods of swimming. They can either 

 swim slowly by the appendages of the abdomen, 

 or jerk themselves suddenly backwards by 

 flexing the tail. They show many interesting 

 points of contrast with crabs and their allies. 

 All the above are called decapod or ten- 

 legged Crustacea, but there are a number of 

 simpler forms also. Thus forms like Asellus 

 in fresh water are represented in the sea by 

 forms like Idotea tncuspidata, which is found 

 on seaweed, especially bladder- wrack, which 

 it closely resembles in colour. Forms like the freshwater 

 shrimp are represented by sandhoppers and many related 

 species. More interesting than these carrion-feeding forms 

 are the species of skeleton-shrimp (Caprella}, found on red 

 weed. The body here is of a very curious shape, and in 

 life the little creatures are the exact colour of the weed 

 over which they climb. The lank body sways in the water 

 i as do the branches of the 



weed, and it takes a sharp eye 

 to discern which is shrimp and 

 which is weed. 



Leaving out of consideration 

 many other forms, we may just 



vi r^~~' mention the little acorn-shells 



which often stud every rock 

 and stone on the shore, and 

 which are small degenerate 



FIG. 74. -The skeleton-shrimp (Caprella). 



I and vi, the first and sixth free thoracic crustaceans. Dead as they 



segments ; ab, the rudimentary abdomen ; look at low tide, when the 

 r/ the respiratory plates by means of water fl()ws each Q its sheU 



which the skeleton shrimp breathes. 1111 



and protrudes slender branched 



feet, by means of which an energetic fishing for food is carried 

 on. The acorn-shells are, however, difficult to keep in con- 

 finement. 



