306 



THE CRUSTACEA 



serves to warn intruders from entering the burrows of these shore- 

 living crabs. In the case of purely aquatic species, the function of 

 these organs is less easy to understand. 



The range of size in Decapoda is greater than in any other 

 group of Crustacea. Some Natantia do not exceed half an inch 

 in length, one Pagurid is adult when 8 mm. long, a species of 

 Porcellanid has a carapace measuring 3 mm. by 5 mm., and some 

 Brachyura are no larger. The largest forms are found among the 



FIG. 183. 



Parapagwrus pilosimanus (Paguridae), lodged in a colony of zoantharian polypes. 

 (From Alcock, Naturalist in Indian Seas.) 



Reptantia ; some Palinuridae and Astacura reach one or even two 

 feet in length and are bulky in proportion. The largest living 

 Arthropod is the Japanese crab Macrocheira (or Kaempferia) Kaempferi, 

 of which the carapace may measure 15 inches in length, and the 

 extended chelipeds of the male may span more than 10 feet. 



PALAEONTOLOGY. 



Fossil remains of Decapods are not known with certainty from 

 any Palaeozoic deposits. Many genera from the Devonian upwards 



