422 TEXT-BOOK OF ZOOLOGY 



distinct animals, from which each of the two "commensals" derives 

 some advantage. 



All the crustaceans hitherto discussed have a well-developed abdomen 

 (tail), and are therefore described as Long-tailed Decapods (Macrura). 

 In contrast with these we have a large number of crustaceans in which 

 the abdomen is very short and turned forwards underneath the cephalo- 

 thorax, which is usually disc-shaped. 



These form the group of Short-tailed Decapods.or Crabs (Brachyura), 

 the most familiar examples of which in our seas are the common Shore 

 Crab (Carcinus mcenas), about \\ inches long, and the larger Edible 

 Crab (Cancer pagurus), about 4£ inches long. 



In these animals the abdomen is rudimentary, and for the most part 

 they progress by walking. Their branchial chambers being tightly 

 closed, many of them are shore - dwellers, being able to live for a 

 considerable time without detriment out of the water. 



Some slow-moving and exclusively aquatic species — like, e.g., the 

 Spider Crab (Maia squinado), common in the Mediterranean and on the 

 west coasts of Europe, and reaching a length of about 7 inches — often 

 resemble walking gardens, their backs being densely covered with a 

 growth of seaweed, sponges, zoophytes, etc. These arrangements afford 

 an excellent protection against enemies lurking in ambush, and also 

 conceal the animals from the prey they are stalking. Several species, 

 indeed, intentionally assume this " disguise," covering their backs with 

 these foreign bodies, whilst some hold such objects constantly over them- 

 selves with the claws of the two last pairs of thoracic legs. 



ORDER II. : MANTIS-SHRIMPS (STOMATOPODA). 



Among the Thoracostraca must also be placed the peculiarly-shaped 

 Mantis Shrimp (Squilla mantis) of the Mediterranean. In this species 

 the carapace, however, does not cover the three last segments of the 

 thorax, while the first two pairs of thoracic legs have been modified into 

 accessory masticatory organs (maxillipedes). The second maxillipedes 

 form powerful raptorial legs (see praying mantis), with which the animal 

 seizes its prey ; whilst the last three similarly-constructed, but smaller, 

 maxillipedes are used for tearing the victim in pieces. (Hence name 

 Stomatopoda, i.e., " mouth-footed.") 



GROUP II: ARTHROSTRACA. 



The crustaceans comprised in this large division are distinguished 

 from the Thoracostraca by the majority of the thoracic segments not 

 coalescing with each other and with the head, and by the absence of a 

 carapace covering these parts. 



