152 



GENERAL ZOOLOGY 



Though the barnacle in its adult condition, as just described, 

 has nothing to fear, in early life the situation is quite differ- 

 ent, for it swims free at the surface of the water in the midst 

 of millions of the young of crabs and other animals. There 

 it is subject to the attacks of animals which might devour it, 

 and many young barnacles are undoubtedly destroyed. At 

 this time it would not be recognized as a barnacle by those 

 who have seen only the adult form. It is characterized by 

 having an unsegmented body, a long upper lip, single median 

 eye, as in Cyclops, and three pairs of jointed locomotor append- 

 ages. This larval form is called a nauplius, and resembles 

 Fig. 78. The barnacle nauplius stage undergoes further com- 

 plicated changes before it attaches itself by the head to some 

 solid object, as a rock, pile, or ship-bottom, when the swim- 

 ming appendages are absorbed and the shell and feathery 

 foot-processes are developed. 



Definition of Crustacea (Lat. crusta, a shell). The crayfish 

 and the species mentioned thus far in this chapter belong 



to the class Crusta'cea. They 

 are constructed on the plan 

 of a series of body-divisions 

 (somites) seldom exceeding 

 twenty in number. In gen- 

 eral, the somites are bilater- 

 ally symmetrical (uniform in 

 structure on either side the 

 median line). Each somite 

 usually has a pair of branched, 

 jointed appendages. As a 

 rule, two pairs of antennre 



are present. A varying number of thoracic somites, in dif- 

 ferent members of the class, are fused to the head. The 

 exoskeleton contains chitin and carbonate of lime. Crustacea 

 are essentially aquatic in their habits and, with the exception 



FIG. 78. Nauplius Stage of Artemia. 

 Much enlarged. (After Joly) 



