308 



NATTTEAIy HISTOET. 



shaped as to be fitted for digging as well as walking; 

 and in others still they are armed with pincers, so as to 

 be instruments of prehension as well as locomotion. In 

 those Crustacea that swim, as Lobsters, Prawns, etc., 

 the abdomen generally ends in a large fin-like expansion, 

 which works up and down in swimming like the tail of 

 the Whale. But iu those which are to walk rather than 

 swim, as the Crab, this part is small, and is bent up un- 

 derneath. 



524. All Crustacea come from eggs. The eggs are 

 commonly carried about adhering to the under part of the 

 abdomen. This we often see in the Lobster. In a boil- 

 ed Lobster they are red, and the mass is called the cor- 

 al. More than twelve thousand eggs have been found 

 attached to the abdomen of a single Lobster. 



525. There is not generally any true metamorphosis in 

 this class. But in some, the animal, when first born, is 

 entirely unlike the perfect animal. This is the case with 



the common Crab. In Fig. 240 you 

 see a representation of the Crab when 

 it first issues from the egg. The 

 large figure is a magnified represent- 

 ation, the natural size being given on 

 the little scroll at the side of it. This 

 is almost as unlike the mature Crab 

 as the larva of the Musquito is unlike 

 the Musquito itself (§ 502). 



526. In most of the Crustacea there 

 is manifest the ring - like arrange- 

 ment of segments which is so char- 

 acteristic of the Articulata (§ 381). 

 But in some it is so much modified 

 as not to be apparent without partio- 

 Thus, in the Crab, as we look on its 

 broad carapace of shell, the ring-like arrangement seems 

 to be entirely forsaken ; but on examining closel)', we 

 find that this carapace is only an excessive enJarge-ment 



Fig. 240.— Early form of 

 the Crab. 



ular observation. 



