CLASS CRUSTACEA. 51 



CLASS CRUSTACEA (krtts ta'she a). 



The CRUSTACEANS often have the integument (in teg'u- 

 ment) hardened by the deposition of limy matter. They 

 are mainly aquatic, and hence breathe by means of gills. 

 There are two pairs of antennae, and the abdomen, as 

 well as the thorax, may bear appendages. Though, on 

 emerging from the egg, the young may resemble, in 

 form, the adult, more often they are born in the Nau- 

 plius (na'pli us) stage, only becoming adults through a 

 series of metamorphoses (met a mor'pho sez). The Nau- 

 plius is of small size, free swimming, but provided with 

 only three pairs of appendages, these finally developing 

 into the two pairs of antennae and the single pair of 

 jaws. 



ORDER COPEPODA (ko pe'po da). 



The common Copepod (ko'pe pod), Cyclops (si'kiops) is 

 a familiar representative of 



FIG. 77. 



the group Entomostraca (en- 

 to mos'tra ka), in which many 

 of the smaller crustaceans, 

 with appendages varying in 

 number, are included. This 

 small white speck is seen 

 swimming with a jerking mo- 

 tion in nearly every tumbler 

 of unfiltered water. If a large 

 specimen be examined with a 



Ct/'dops commnn'is. Nauplius of Cyclops. 



hand lens, a pair of sacs, filled 



with eggs, will be found to hang from each side of the 

 abdomen. On hatching from these eggs, the young are 

 in the "nauplius stage." 



