202 



Arthropoda. Glass 1. Crustacea. 



different forms, are in the direction of clumsiness, deformity, and im- 

 mobility; tlie abdomoQ degenerates, the segmentation is obliterated. 



Fig. 164. Various parasitic Copepoda : 1 Nogagus borealis, male from the ventral 

 side. 2 Oaligus rapax, female from above. 3 Chondracanthus gihhosug, female from 

 below ( (? the male). 4 Brachiella thynni, female. 5 Male of the same species (more 

 enlarged), a^ — a^ first and second antennae, / oandal appendages, hk second maxilla, 

 kf maxilliped, p^ — p^ first — fourth pairs of legs, o egg-sac. — 1, 2, 4, 5 after St enstrup and 

 Ltitken, 3 after Claus. 



those limbs, which do not serye as organs of adhesion, atrophy, or 

 become functionless ; this applies especially to the true legs, which 



are either wanting, have 

 ■^ S G degenerated to mere ves- 



tiges (Fig. 165£), or have 

 become large thick ap- 

 pendages, without setee, 

 exhibiting only feeble in- 

 dications of their original 

 form. Often such parasitic 

 Crustacea are provided 

 with peculiar outgrowths, 

 rendering their appearance 

 still more striking. As a 

 rule they are blind. Where 

 the reduction is most ad- 

 vanced, the entire animal 



Fig. 165. A, Penella sagitta (parasitic on certain 

 Fish), 9 , natural size. B anterior end enlarged, 

 y' first, p' fourth pair of legs, o egg-sac. C Rerpyl- 

 lohius arcticus (parasitic on Chsetopods), 9 enlarged, 

 egg-sac. The irregularly lobed part is sunk in the 

 body of the host. — After Steenstrup and Liitken. 



