SECT. vii. DECAPOD CRUSTACEA. 189 



Macrura. 



The body of the Macrura, or long-tailed crustaceans, 

 consists of a number of segments or rings joined end to 

 end, having jointed members on each side. Every indi- 

 vidual joint is covered with a hard crust to afford 

 support to the muscles. A certain number of the rings, 

 which form the tail, are always distinct, similar, and 

 movable on one another, whilst the remainder, which 

 form the carapace or shell, are confluent so as entirely to 

 obliterate the divisions. But generally the arrangement 

 of these twenty-one rings is such that seven of them are 

 confluent and form the head, seven confluent rings form 

 the thorax or throat, and the seven non-confluent rings 

 form the tail. In the Decapods the three last head 

 rings greatly expanded are cemented to those of the 

 thorax, so as to form the carapace or shell, which covers 

 all the body of the animal except the tail. This struc- 

 ture may be traced on the under-surface of the crab. 



A ring consists of an upper and an under arch, with 

 a space between them, so as to let the feet and other 

 appendages pass through. In the long-tailed tribe the 

 tail is bent and unbent by muscles attached to the under 

 and upper surfaces of each ring, which give the tail a 

 powerful motive force, for, by bending it suddenly under 

 the body, and then as suddenly stretching it out, the 

 animal darts backwards through the water. 



The Decapods have five pairs of walking feet ; the 

 front pair are claws employed to seize their prey, and 

 occasionally for walking ; the other four pairs are cylin- 

 drical, and end in sharp hooked points. 



Brachyura. 



The Brachyura surpass all the other Decapods in 

 compactness and concentration, and are without excep- 



