THE DECAPOD CRUSTACEA. 



151 



On the other hand, the structure of the external parts of a 

 Crustacean in the general case determines the systematic 

 position of the animal, and the examination of such external 

 parts requires more care than anatomical skill in the strict 

 sense. The Crustacea are therefore par excellence the class 

 for the novice, the one above all others in which he can 

 hope to walk by sight and not by faith. 



In studying the Crustacea it is convenient to begin with 

 the higher forms, which are usually of such size as to make 

 observation easy. To acquire a general knowledge of the 

 structure, we may compare three common forms a prawn, 

 a lobster, and a crab. The common prawn (Palcemon serratus), 

 a beautiful little creature about four inches long, is not likely 

 to be found on the East Coast, but a smaller species (P. 

 squilla) is not uncommon in rock pools, and is large enough 

 for our purpose. The hump-backed Esop prawn (Pandalus 



annulicornis) may 

 also be found far 

 out on the rocks; 

 while, failing all 

 three, the common 

 shrimp may be sub- 

 stituted. As to the 

 second specimen, 

 the lobsters really 

 lie somewhat out- 

 side our province, 

 but the Norway 

 lobster (Nephrops 

 norvegicus) can be 

 purchased very cheaply at a fishmonger's, and is admirably 

 adapted for the study of many Crustacean characters. Those 

 who do not find it available will probably be able to obtain 

 the fresh-water crayfish, or that somewhat costly luxury 

 the true lobster. Add to your specimens the common shore 

 crab or the edible crab, and you are prepared for the study 

 of the characters of the Crustacea. 



Place your three specimens prawn or shrimp, lobster or 

 crayfish, and crab side by side, and note first their common 

 characters. All three can be divided into two similar parts 

 by a line down the middle of the body that is, all are 



FIG. 47. A common prawn (Palcemon squilla). 



