THE CYCLE OF LIFE 451 



another of a difierent pattern ; it undergoes a metamor- 

 phosis, fasting all the time, and becomes a miniature 

 adult with its beautifully waving curl-hke appendages — 

 comparable, as Huxley said, to a shrimp fixed head down- 

 wards and back downwards to a rock, and kicking its 

 food into its mouth with its legs. 



Shore Crab. — ^No one could suspect from an obser- 

 vation of a common shore-crab, such as Carcinus mosnas, 

 that its early youth was spent in open waters. The larva 

 is a minute transparent free-swimming creature, known 

 as a zosea, with its tail sticking out in a line with the rest 

 of the body, with eight pairs of hmbs instead of the adult's 

 total of at least twice as many, and with a curved spine 

 arising from the middle of the cephalothorax shield. This 

 httle animal feeds and grows and moults its cuticle, and 

 feeds and grows and moults again, becoming eventually 

 a second larval form, known as the Megalops. This has 

 lost the spine and gained a broader body and also additional 

 hmbs, namely those corresponding to the forceps and 

 walking legs of the adult crab. But its tail is still sticking 

 out in a line with the rest of the body. The Megalops feeds 

 and grows and moults, gets its tail tucked forwards under 

 the cephalothorax, and becomes a miniature crab about 

 the size of a quarter of one's httle finger nail — a creature no 

 longer suited for free swimming, but for the floor of the 

 sea in shallow water, whence it creeps up on to the 

 shore. 



Freshwater Insects. — There is something pecuharly 

 fascinating in the hfe-histories of freshwater insects, partly 

 because of the sharp contrast between the aquatic and 

 the aerial chapters, partly because of the subtlety of the 

 adaptations to hfe in the water. Every one has enjoyed 



