412 



CHARACTERS OF INVERTEBRATE ANIMALS 



Lobster (Palinurus vulgaris) (fig. 251), Norway Lobster (Nephrcps 

 Norvegicus), Prawn (Pal&mon serratus\ Shrimp (Crangon vul- 

 garis]. The Crayfish (Astacus fluviatilis] is a fresh-water form. 



Fig. 251. K ode-Lobster (Falinurzts vulgaris) 



Another section, including forms intermediate between the 

 above and Crab, is formed by the Hermit-Crabs (fig. 252), which 



take up their abode in the empty shells of 

 various sea-snails, and possess a soft un- 

 symmetrical tail that has lost most of its 

 appendages except the last pair. These 

 have lost their original function of acting 

 as swimming organs, and are modified 

 into hook-like structures, by which the 

 hermit-crab holds on to its house. 



The last section of the Decapods 

 includes the true Crabs (fig. 250), in which 

 the cephalo- thorax is very broad, and the 

 tail so much reduced as to be useless 

 as a swimming organ. Among British 

 species may be noted the Edible Crab 

 ( Cancer pagurus) and the Shore Crab (Carcinus mcenas). 



2. The Opossum- Shrimps (Schizopoda) are small marine forms 



Fig. 252. A Hermit-Crab (Pagurus 

 Bernkardus), removed from its dwelling 



