CRUSTACEA. 



221 



the ducts of the reproductive organs open to the exterior 

 in the thoracic region, never 

 in the abdomen. In almost 

 all forms the eggs are carried 

 about by the mother until they 

 are hatched. In almost all 

 the lower Crustacea the young 

 escapes from the egg in a very 

 immature condition, known as 

 a Nauplius (fig. 49), a name 

 given years ago under the be- 

 lief that it was an adult. The 

 nauplius has an unsegmented 

 body, a single median eye, and 

 only three pairs of appendages 

 -antennulae, antennae, and 

 mandibles the antennulse being solely sensory, while 

 antennae and mandibles are used in both swimming and 

 eating. In the higher Crustacea the nauplius stage is 

 passed in the egg, and the young hatches in a more ad- 

 vanced condition sometimes closely like the adult in all 

 except size. Growth is allowed for by frequent molts 

 of the external cuticle of the body. 



Over 10,000 species of Crustacea are known, almost all 

 of them aquatic, and the majority marine. Only a few, 

 like the sow-bugs and land-crabs, live on the land. A few 

 are vegetarians, some are parasites on other animals, but 

 the majority are scavengers, feeding on decaying organic 

 matter. The Crustacea may be conveniently divided into 

 two subclasses: Mala.costraca and Entomostraca. 



FIG. 49. Nauplius stage of fairy- 

 shrimp (Branchipus). After 

 Claus. 



