ARTHROPODS 187 



Food-getting. The food of the Crustacea is usually animal, 

 either alive or dead, some even being cannibals, while others act 

 as useful scavengers. A few of the smaller forms are peaceful 

 vegetarians. Their swiftness, claws, mouth parts, color protection, 

 and sense of touch and smell all are adaptations for food-getting 

 and their large number shows how well able they are to cope with 

 their surroundings. 



Life History. The eggs, which often number thousands, are laid 

 by the female, fertilized by sperms from the male as they are laid, 

 and attached to the swimmerets where they are carried and pro- 

 tected by the mother for about ten months. The young after hatch- 

 ing, which occurs in summer, cling to the swimmerets for some time. 

 When first hatched they are very small, not entirely like the adult 

 in structure, and they remain at the surface of the water for the 

 first stages. After moulting four or five times, they settle down on 

 the bottom among the rocks, where they live on smaller crus- 

 taceans. During these early stages which occupy ten to fifteen 

 days they are nearly defenceless and millions are destroyed by 

 other aquatic animals for food. After reaching the bottom they 

 are somewhat better protected though still destroyed in large 

 numbers. This high mortality is the reason for the production of 

 such large numbers of eggs. During their life at the bottom, 

 moulting occurs at longer intervals until adult size is reached at 

 the age of five years (in case of the lobster) after which they do not 

 moult oftener than once in one or two years. 



Moulting. This moulting, or shedding of the exo-skeleton is a 

 direct result of having the hard parts outside. They cannot grow 

 larger except by shedding their armor, and this is a point in which 

 the internal skeleton of the higher animals is a very great ad- 

 vantage. With it, growth may be continuous. However, the exo- 

 skeleton provides better protection. When ready to moult, the lime 

 is partly absorbed from the skeleton; the carapace splits along the 

 back, water is withdrawn from the tissues which makes them 

 smaller and the animal literally humps itself out of its former 

 skeleton, leaving behind the lining of its stomach and its teeth. 

 Immediately water is absorbed and growth proceeds very rapidly. 



