6572 



Crustacea, 



when it does swim its organs of progression are not those of its 

 younger days. In its first infancy the tail had no appendages ; now it 

 has five pairs of plates fringed with hairs; these are the small oars 

 with which at this time it navigates its course in its occasional passages 

 through the water ; but its more congenial position is in some sheltered 

 nook among the stones and weed at the bottom of the sea. Age is 

 creeping on, so it puts off its youthful Zoe habits and assumes those 

 becoming a grave and sober crab. 



We said that in the hot and quiet days in early summer these young 

 Crustacea may be found floating near the surface of the sea ; but they 

 know no rest, — they swim by night as well as day, for in the calm 

 nights, when the air is warm and pleasant, the ocean shines with 

 myriads of its own native stars. Break the surface with the dip of the 

 oar, and it drops with a soft, sweet light, and every spark a living 

 atom : among this resplendent glow the living Crustacea form no in- 

 considerable portion. Dip but a small quantity of the water, and 

 carry home for observation, and small Entomostracan Crustacea in 

 abundance, both living and dead, cast-off skins and broken limbs, 

 testify to the number of the class that are present in the water. 



The Zoe has ceased to be one, — it has lost its fantastic shape ; the 

 horn upon its back and brow are gone — lost in the greater develop- 

 ment of the surrounding parts ; the narrow sides of the animal swell 



out, the eyes are smaller and more 

 in proportion, and the tail ceases to 

 be forked. The creature has now 

 become a Megalopa. This name 

 was given by Dr. Leach to what 

 he believed to be a new genus of 

 Crustacea, which contained two 

 species ; but this genus as well as 

 both species that it contains are 

 now known to be but stages in the 

 progressive growth of the same 

 animal : the first of these that was found was taken by Colonel 

 Montagu amongst corallines on the back of the prickly crab {Mala 

 Squinado), on the southern coast of Devon, and named after the finder; 

 the other, which is the younger form of the two, is called Megalopa 

 armata, from the great spine upon its back; it was found in a crab-pot 

 in Bigbury Bay, by Mr. C. Piideaux. Both of them are figured in 

 Dr. Leach's work on the British crabs. 



When Mr. Thomson first astonished naturalists, the wonder was 



Fig. 5. — Young crab in the intermediate stage. 



