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The Study of Animal Life part m 



to the complex, from the general to the special, we must 

 be careful to notice that he did not say that the young 

 mammal was once like a little fish, afterwards like a reptile, 

 and so on ; he compared the embryo mammal at one stage 

 with the embryo fish, at another stage with the embryo 

 reptile, which is a very different matter. 



Fig. 38. — Embryos of fowl, a ; dog, h \ man, c. (From Chambers's Encyclop. ; 

 after Haeckel.) 



Fritz Mijller, in his Facts for Darwin, illustrated the 

 same idea in relation to Crustacea. When a young cray- 

 fish is hatched, it is practically a miniature adult. When 

 a young lobster is hatched, it differs not a little from the 

 adult, and is described as being at a Mysis stage, — Mysis 

 being a prawn-like crustacean. It grows and moults and 

 becomes a little lobster. When a crab is hatched, it is 

 quite unlike the adult, it is liker one of the humblest 

 Crustacea such as the common water-flea Cyclops, and is 

 described as a Zoea. This Zoea grows and moults and 

 becomes, not yet a crab but a prawn-like animal with ex- 

 tended tail, a stage known as the Megalopa. This grows and 

 moults, tucks in its tail, and becomes a young crab. And 

 again, when the shrimp-like crustacean, known as Penceus, 

 is hatched, it is simpler than any known crustacean, it is 

 an unringed somewhat shield -shaped little creature with 

 three pairs of appendages and a median eye. It is known 

 as a Nauplius and resembles the larva of most of the simpler 

 crustaceans. It grows and moults and becomes a Zoea, 

 grows and moults and becomes a Mysis, grows and moults 

 and becomes a Penceus. 



