406 COPEPODA. 



take place. First of all a pair of appendages arises behind the mandi- 

 bles, 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 num- 

 ber 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 antenna?, 

 mandibles, maxillae, the now double pair of maxillipeds (derived from 

 the original single pair of appendages), and first pair of thoracic appen- 

 dages (|)') are attached. The second pair of thoracic appendages (p 2 ) is 

 fixed to the first free segment, and the rudiment of a third pair (p 3 ) pro- 

 jects 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 develop- 

 ment, 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 

 large number of secondary characters. So far as is known a more or less 

 modified Nauplitis larva is usually preserved. 



The development of Achtheres percarurn, one of the Lernseopoda parasitic 

 in the mouth, etc. of the common Perch, may be selected to illustrate the 

 mode of development of these forms. The larva leaves the egg as a much 

 simplified Nanplius (fig. 230 A). It has an oval body with only the two 

 anterior pairs of Nauplius appendages ; both of them in the rudimentary 

 condition of un jointed rods. The usual median eye is present, and there is 

 also found a peculiar sternal papilla, on which opens a spiral canal filled 

 with a glutinous material, which is probably derived from a gland which 

 disappears on the completion of the duct. The probable function of this 

 organ is to assist at a later period in the attachment of the parasite to 



