CRUSTACEA. 



321 



and have natatory extremities developed from the five posterior 

 rings. 



In the lobster (Astacus Marinus) we find not only the cephalic 

 segments anchylosed together, but those of the thorax also ; and al- 

 though the lines of demarcation between them are still recognisable 

 upon the ventral aspect of the body, superiorly the entire thorax 

 and head are consolidated into one great shield (cephalo-thorax) , 

 the abdominal segments only remaining distinct and moveable. 



In the Crabs the centralization of the external skeleton is carried 

 to still greater lengths, so as to enable this tribe of Crustaceans to 

 become more or less capable of leaving their native element, and 

 walking upon the shores of the sea, or even in some instances of 

 leading a terrestrial existence, as in the case of the land- crab of the 

 West India islands. The abdominal segments, however, still re- 

 main free, though proportionately of very small dimensions ; and, 

 being no longer useful in swimming, the abdomen is folded beneath 

 the enormously developed thoracic portion of the body. 



In the King-Crab (Limulus Polyphemus; Jig. 151) even the 

 divisions of the abdomen are obli- Fig. 151. 



terated, the whole body being co- 

 vered by two enormous shields, and 

 the tail prolonged into a formidable 

 serrated spine, of such density and 

 sharpness that in the hands of sa- 

 vages it becomes a dreadful weapon, 

 and is used to point their spears 

 either for the chase or war. 



The reader will at once perceive 

 the strict parallelism that may be 

 traced between the changes which 

 occur during* the metamorphosis of 

 insects, and those observable as we 

 thus advance from the lowest to 

 the most highly organized Crusta- 

 cean genera; and even the steps 

 whereby we pass from the Anneli- 

 dan to the Myriapod, and from 

 thence to the Insect, the Scorpion, and the Spider, seem to be re- 

 peated as we thus review the progressive developement of the class 

 before us. 



Having thus found that the annuli, or rings, which compose the 



Y 



