THE DECAPOD CRUSTACEA. 153 



obviously segmented region, covered by a shield, and a tail 

 made up of a succession of similar parts. The anterior 

 region we call the cephalothorax for it is made of head and 

 thorax united the posterior, the abdomen or tail. The 

 cephalothorax, or united head and body, contains the greater 

 part of the organs of the body ; the tail is mainly filled up 

 by powerful muscles (flesh), and in both prawn and lobster 

 serves as an organ of locomotion. The crab, on the other 

 hand, differs markedly from the other two in that it appears 

 to have no tail. Turn your crab over on its back, however, 

 and you will have no difficulty in seeing that it has really a 



FIG. 49. Shore crab (Carcinus mcenas). 



true tail, reduced in size, useless for locomotion, without 

 muscles, and habitually carried reflexed on the body, but a 

 tail none the less. The body of the crab, no less than that 

 of prawn or lobster, consists of cephalothorax and abdomen, 

 but the proportions of the two parts differ markedly. In 

 consequence of this marked difference the order of Crustacea 

 to which the three forms belong (Decapoda, or forms with 

 ten legs) is often divided into long-tailed forms, such as 

 shrimp, prawn, and lobster, and short- tailed forms, such as 

 crabs. 



Although there is considerable resemblance between 

 prawn and lobster as contrasted with crab, a little more 



