124 NEW VOKK STATE MUSEUM 



modifications are carried out in every direction and form the basis 

 for the division into orders, families, genera and species. 



It is certain too that in no other group of animals are there such 

 wide deviations from what may be regarded as a typical form, such 

 as the second one described above, or such extreme modifications in 

 adaptation to particular or special modes of life. It may even be 

 carried to the extent of the entire loss of all crustacean characters, 

 as in Sacculina, a parasite on a crab, where in the adult there is no 

 sign of segmentation, no appendages, no mouth, just a sac with a 

 number of rootlike filaments extending through the body of the host. 

 It is only by its developmental history that we know it to be a 

 crustacean. 



This variation takes place to a certain extent even within the 

 groups of closely related forms which are regarded as a tribe or a 

 family and explains the frequent use of the words " usually " or 

 " generally " in the diagnoses, or the terms " with or without " a 

 certain part. 



The sexes in the Crustacea are usually separate, and the eggs are 

 carried by the female either in a brood pouch or attached to the 

 limbs. The young, in most cases, hatch in a free swimming form 

 so unlike the parent that formerly they were regarded as distinct 

 species. This larva grows by a series of molts whereby it gradu- 

 ally increases in size and attains the adult form. In other words 

 there is indirect development or a metamorphosis. 



In other cases, such as in the crayfishes and the Arthrostaca, the 

 young are very like the parent, differing only in small details, and 

 the development is direct. 



Owing to the hardness of the integument, which is incapable of 

 growth, increase in size in the Crustacea is obtained by the shedding 

 of the hard shell and the formation of a new one underneath. This 

 new one is soft and thus allows a certain increase in size, but it soon 

 hardens. A familiar example of this soft condition after molting 

 is seen in the " soft shell crab." 



The Crustacea as a class may be regarded as holding a very similar 

 place in the economy of the ocean to that of the insects on the land. 

 Like the insects, they are widely distributed, hardly any place being 



