ANIMAL META:\rOIlPHOSIS. 175 



enclosed in a transparent, but rather thick, shell, composed of two 

 convex valves united by a straight hinge. These valves are sym- 

 metrical in size and shape, so that the shell resembles that of a 

 cockle more than an adult oyster. Sooner or later the young 

 oysters lose their ciliated disc, settle down in life, fix themselves 

 to some solid body, the upper shell remains convex while the 

 lower becomes flat, and the animal acquires the asymmetrical 

 characteristics of the adult. 



The young Anodon, or Fresh-water Mussel (Fig. 14), is 

 enclosed in a pair of three-sided valves, which are furnished with 

 curious barbed or serrated hooks, by means of which it attaches 

 itself to the fins and gills of small fish ; and so different are the 

 young in appearance from the adult mussels that they were 

 looked upon as distinct animals and described under the name 

 of Glochidia. 



In the Cephalopoda (including the Octopi and Cuttle-fish) 

 development is direct, no metamorphosis taking place after they 

 leave the egg. 



The next sub-kingdom — the Arthropoda — contains the 

 Crustacea and the Insecta, the young of most of which pass 

 through a metamorphosis. 



The Crustacea include the lobsters, crabs, shrimps, entomos- 

 traca, barnacles, and acorn-shells. The young lobster only 

 differs from the adult in the possession of small outer appendages 

 on the walking-limbs and in the smaller size of the tail ; but the 

 crab emerges from the egg in a form totally unlike the adult, as a 

 little swimming creature, about a quarter of a line in length, with 

 a dorsal shield armed with a strong median spine, and followed 

 by a jointed post-abdomen, which bears no appended limbs. It 

 is now known as a Zoaea (Fig. 16). In seven or eight days, 

 having cast off his coat several times, he loses his spine, his back 

 becomes broader and more depressed ; the eyes, from being 

 sessile, are elevated on moveable stalks; the claws undergo an entire 

 revolution, the first pair becoming stouter than the others and 

 armed with strong nippers, and he becomes a tailed crab (Fig. 17), 

 although the tail is greatly diminished in size. vStill, he goes on 

 swimming, and after moulting a few times more his tail is folded 

 under, and he sinks to the bottom a true walking-crab. 



