THE JOINTED-FOOT ANIMALS 153 



of the lowest forms, breathe through gills. Nine tenths of 

 the class are said to live in the ocean, some in fresh water, 

 and relatively few on land. 



As in the insects, the hard exoskeleton necessitates fre- 

 quent molts to provide for growth, and also, as in that class, 

 growth is sometimes accompanied 

 by marked metamorphosis after 

 hatching. The lower forms, such 

 as Cyclops, the barnacles, and the 

 parasitic Crustacea, hatch as the 

 peculiar nauplius type of larva 

 already mentioned in the descrip- 

 tion of the rock-barnacle and illus- 

 trated in Fig. 78. FIG. 79. Zoea Stage of Crab. 



In the higher Crustacea the Much enlarged: (After 

 .. . Emerton) 



crabs pass the nauplius stage in 



the egg and hatch in the zoea stage (Fig. 79). The lobster 

 passes the nauplius and the zoea stage in the egg, and hatches 

 in the my sis stage (Fig. 70). The crayfish passes all three 

 stages in the egg, and hatches practically in the form of the 

 adult, the young growing without subsequent metamorpho- 

 sis. It is important to note that the stages nauplius, zoea, and 

 mysis, in the development of the higher Crustacea, are practi- 

 cally identical with the adult form of certain other Crustacea 

 which are considered to be more primitive in organization. 

 The fact that the lower type form reappears in the develop- 

 ment of the higher is often cited as evidence in favor of the 

 recapitulation theory referred to in the chapter on insects. 



THE TRILOBITES 



Some of the most abundant of the forms in the earlier 

 periods of life on the earth were the Tri'lobites (Pha'cops 

 cauda'tus, Fig. 80), which are now considered to be closely 



