Crustacea. 



6573 



created by the statement that the animals underwent a distinct meta- 

 morphosis, — that the Zoe changed to a Megalopa and the Megalopa 

 became a crab, just in the same way that a caterpillar becomes a grub 

 and the grub passes into a butterfly ; and this idea has been taught by 

 implication ever since in the most important works upon the subject. 

 But observations upon many specimens taken at different ages have 

 proved that these different stages are but mile-stones in the passage, 

 and are arrived at by a series of progressive steps, each one scarcely 

 appreciable from that which has preceded it. 



Most young naturalists look for wonders: things that are astonishing 

 are pleasing to the fancy. The Nautilus, when its history was dressed 

 in the poet's fable, was a prettier thing than the poor cheat it has been 

 found out to be. So with the crab, truth is not so pleasant as fiction, and 

 there was something astonishing in these insects of the deep fulfilling 

 in their history the conditions similar to their supposed analogues of 

 the land. 



Successive moults still bring the animal nearer to the form of the 

 parent: the long projecting rostrum disappears, and the postorbital 

 region, the position in which the liver is developed, increases in 

 proportion, the tail is bent close beneath, and the animal assumes a 

 square form ; the continuing growth of the liver region increases the 

 lateral dimensions of the crab, so that the adult is found broader than 

 it is long. But all these alterations are slow, and without any sudden 

 change, any metamorphosis, such as appears to exist in insects.* 



The young crab is no longer a swimmer: those who wish to find 

 it now must search the rocky pools upon the sea-shore, and turn the 

 stones and weed, and seek beneath their sheltering protection, where, 

 half-buried in the soil, it hides from the presence of its enemies, and 

 watches for prey. 



Although there is no sudden metamorphosis, there is a great change 

 of form between the early larva and the adult Crustacea, and the same 

 law r of development appears to be persistent, as in animals of the 

 higher types. The progressive growth of the creature passes through 

 many stages, which have their types in adult forms : these would 

 seem to suggest that each may be but an arrest in the development of 

 the more perfect animal. The entomostracous form is that which it 

 most resembles at first, both in its general appearance and in its move- 

 ments and habits ; with its growth it assimilates to those of a higher 



* The development of the butterfly, &c. is quite as gradual, but it is hidden by 

 the external forms of the caterpillar and grub. 



