CRUSTACEA. 489 



changes take place. First of all a pair of appendages arises 

 behind the mandibles, which form the maxillae (fig. 229 B) ; at 

 the same time the basal joint of the maxillae develops a cutting- 

 blade. Three successive pairs of appendages (fig. 229 C) next 

 become formed the so-called maxillipeds (the homologues of 

 the second pair of maxillae), and the two first thoracic limbs. 

 Each of these though very rudimentary is nevertheless bifid. 

 The body becomes greatly elongated, and the caudal fork more 

 developed. 



Up to this stage of development the Nauplius appendages 

 have retained their primitive character almost unaltered ; but 

 after a few more ecdyses a sudden change takes place ; a cephalo- 

 thoracic shield becomes fully developed, and the larva comes to 

 resemble in character an adult Copepod, from which it mainly 

 differs in the smaller number of segments and appendages. In 

 the earliest 'Cyclops' stage the same number of appendages are 

 present as in the last Nauplius stage. There (fig. 229 D) is a 

 well developed cephalo-thorax, and four free segments behind it. 

 To the cephalo-thoracic region the antennas, mandibles, maxillae, 

 the now double pair of maxillipeds (derived from the original 

 single pair of appendages), and first pair of thoracic appendages 

 (p l ) are attached. The second pair of thoracic appendages (/ 2 ) 

 is fixed to the first free segment, and the rudiment of a third 

 pair (/ 3 ) projects from the second free segment. The first pair 

 of antennae has grown longer by the addition of new joints, and 

 continues to increase in length in the following ecdyses till it 

 attains its full adult development, and then forms the chief 

 organ of locomotion. The second pair of antennae is much 

 reduced and has lost one of its rami. The two rami of the 

 mandibles are reduced to a simple palp, while the blade has 

 assumed its full importance. The maxillae and following appen- 

 dages have greatly increased in size. They are all biramous, 

 though the two rami are not as yet jointed. The adult state is 

 gradually attained after a number of successive ecdyses, at 

 which new segments and appendages are added, while new 

 joints are formed for those already present. 



Parasita. The earliest developmental stages of the parasitic types 

 of Copepoda closely resemble those of the free forms, but, as might be 

 expected from the peculiarly modified forms of the adult, they present a 



