198 The Study of Animal Life PART in 



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, b \ man, c. (From Chambers's Encyclop. \ 

 after Haeckel.) 



Fritz Miiller, 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 Pcnceus, 

 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 larvae 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 Penczus, 



